Category: philosophy

Aristotle (Ancient Greek: Ἀριστοτέλης, romanized: Aristotélēs; 384–322 BC) was an ancient Greek philosopher and polymath. His writings span the natural sciences, philosophy, linguistics, economics, politics, psychology, and the arts. As the founder of the Peripatetic school of philosophy in the Lyceum in Athens, he began the wider Aristotelian tradition that followed, which set the groundwork for the development of modern science. Little is known about Aristotle's life. He was born in the city of Stagira in Chalkidiki (northern Greece) during the Classical period. His father, Nicomachus, died when Aristotle was a child, and he was brought up by a guardian. At around eighteen years old, he joined Plato's Academy in Athens and remained there until the age of thirty seven (c. 347 BC). Shortly after Plato died, Aristotle left Athens and, at the request of Philip II of Macedon, tutored his son Alexander the Great beginning in 343 BC. He established a library in the Lyceum, which helped him to produce many of his hundreds of books on papyrus scrolls. Though Aristotle wrote many treatises and dialogues for publication, only around a third of his original output has survived, none of it intended for publication. Aristotle provided a complex synthesis of the various philosophies existing prior to him. His teachings and methods of inquiry have had a significant impact across the world, and remain a subject of contemporary philosophical discussion. Aristotle's views profoundly shaped medieval scholarship. The influence of his physical science extended from late antiquity and the Early Middle Ages into the Renaissance, and was not replaced systematically until the Enlightenment and theories such as classical mechanics were developed. He influenced Judeo-Islamic philosophies during the Middle Ages, as well as Christian theology, especially the Neoplatonism of the Early Church and the scholastic tradition of the Catholic Church. Aristotle was revered among medieval Muslim scholars as "The First Teacher", and among medieval Christians like Thomas Aquinas as simply "The Philosopher". He has been referred to as the first scientist. His works contain the earliest known systematic study of logic, and were studied by medieval scholars such as Peter Abelard and Jean Buridan. His influence on logic continued well into the 19th century. In addition, his ethics, although always influential, has gained renewed interest with the modern advent of virtue ethics.

Consciousness is being aware of something internal to one's self, or of states or objects in one's external environment. It has been the topic of extensive explanations, analyses, and debate among philosophers, scientists, and theologians for millennia. There is no consensus on what exactly needs to be studied, or whether consciousness can be considered a scientific concept. In some explanations it is synonymous with mind, while in others it is considered an aspect of it. In the past, consciousness meant one's "inner life": the world of introspection, private thought, imagination, and volition. Today, it often includes various forms of cognition, experience, feeling, or perception. It may be awareness, awareness of awareness, metacognition, or self-awareness, either continuously changing or not. There is also a medical definition that helps, for example, to discern "coma" from other states. The disparate range of research, notions, and speculations raises some curiosity about whether the right questions are being asked. Examples of the range of descriptions, definitions and explanations are: ordered distinction between self and environment, simple wakefulness, one's sense of selfhood or soul explored by "looking within", being a metaphorical "stream" of contents, or being a mental state, mental event, or mental process of the brain.

Democracy is a form of government in which political power is vested in the people or the population of a state. Under a minimalist definition of democracy, rulers are elected through competitive elections while more expansive or maximalist definitions link democracy to guarantees of civil liberties and human rights in addition to competitive elections. In a direct democracy, the people have the direct authority to deliberate and decide legislation. In a representative democracy, the people choose governing officials through elections to do so. The definition of "the people" and the ways authority is shared among them or delegated by them have changed over time and at varying rates in different countries. Features of democracy often include freedom of assembly, association, personal property, freedom of religion and speech, citizenship, consent of the governed, voting rights, freedom from unwarranted governmental deprivation of the right to life and liberty, and minority rights. The notion of democracy has evolved considerably over time. Throughout history, one can find evidence of direct democracy, in which communities make decisions through popular assembly. Today, the dominant form of democracy is representative democracy, where citizens elect government officials to govern on their behalf such as in a parliamentary or presidential democracy. In the common variant of liberal democracy, the powers of the majority are exercised within the framework of a representative democracy, but a constitution and supreme court limit the majority and protect the minority—usually through securing the enjoyment by all of certain individual rights, such as freedom of speech or freedom of association. The term appeared in the 5th century BC in Greek city-states, notably Classical Athens, to mean "rule of the people", in contrast to aristocracy (ἀριστοκρατία, aristokratía), meaning "rule of an elite". In virtually all democratic governments throughout ancient and modern history, democratic citizenship was initially restricted to an elite class, which was later extended to all adult citizens. In most modern democracies, this was achieved through the suffrage movements of the 19th and 20th centuries. Democracy contrasts with forms of government where power is not vested in the general population of a state, such as authoritarian systems. Historically a rare and vulnerable form of government, democratic systems of government have become more prevalent since the 19th century, in particular with various waves of democratization. Democracy garners considerable legitimacy in the modern world, as public opinion across regions tends to strongly favor democratic systems of government relative to alternatives, and as even authoritarian states try to present themselves as democratic. Democracy more consistently results in improved health, education and economic outcomes. According to the V-Dem Democracy indices and The Economist Democracy Index, less than half the world's population lives in a democracy as of 2022. At the same time, while representative democracy is widely valued, Pew Research Center found that dissatisfaction with democratic performance is common even in many established democracies.

Epistemology is the branch of philosophy that examines the nature, origin, and limits of knowledge. Also called the theory of knowledge, it explores different types of knowledge, such as propositional knowledge about facts, practical knowledge in the form of skills, and knowledge by acquaintance as a familiarity through experience. Epistemologists study the concepts of belief, truth, and justification to understand the nature of knowledge. To discover how knowledge arises, they investigate sources of justification, such as perception, introspection, memory, reason, and testimony. The school of skepticism questions the human ability to attain knowledge, while fallibilism says that knowledge is never certain. Empiricists hold that all knowledge comes from sense experience, whereas rationalists believe that some knowledge does not depend on it. Coherentists argue that a belief is justified if it is consistent with other beliefs. Foundationalists, by contrast, maintain that the justification of basic beliefs does not depend on other beliefs. Internalism and externalism debate whether justification is determined solely by mental states or also by external circumstances. Separate branches of epistemology focus on knowledge in specific fields, like scientific, mathematical, moral, and religious knowledge. Naturalized epistemology relies on empirical methods and discoveries, whereas formal epistemology uses formal tools from logic. Social epistemology investigates the communal aspect of knowledge, and historical epistemology examines its historical conditions. Epistemology is closely related to psychology, which infers the beliefs people hold from their words and actions, while epistemology studies the norms governing the evaluation of beliefs. It also intersects with fields such as decision theory, education, and anthropology. Early reflections on the nature, sources, and scope of knowledge are found in ancient Greek, Indian, and Chinese philosophy. The relation between reason and faith was a central topic in the medieval period. The modern era was characterized by the contrasting perspectives of empiricism and rationalism. Epistemologists in the 20th century examined the components, structure, and value of knowledge while integrating insights from the natural sciences and linguistics.

Ethics is the philosophical study of moral phenomena. Also called moral philosophy, it investigates normative questions about what people ought to do or which behavior is morally right. Its main branches include normative ethics, applied ethics, and metaethics. Normative ethics aims to find general principles that govern how people should act. Applied ethics examines concrete ethical problems in real-life situations, such as abortion, treatment of animals, and business practices. Metaethics explores the underlying assumptions and concepts of ethics. It asks whether there are objective moral facts, how moral knowledge is possible, and how moral judgments motivate people. Influential normative theories are consequentialism, deontology, and virtue ethics. According to consequentialists, an act is right if it leads to the best consequences. Deontologists focus on acts themselves, saying that they must adhere to duties, like telling the truth and keeping promises. Virtue ethics sees the manifestation of virtues, like courage and compassion, as the fundamental principle of morality. Ethics is closely connected to value theory, which studies the nature and types of value, like the contrast between intrinsic and instrumental value. Moral psychology is a related empirical field and investigates psychological processes involved in morality, such as reasoning and the formation of character. Descriptive ethics describes the dominant moral codes and beliefs in different societies and considers their historical dimension. The history of ethics started in the ancient period with the development of ethical principles and theories in ancient Egypt, India, China, and Greece. This period saw the emergence of ethical teachings associated with Hinduism, Buddhism, Confucianism, Daoism, and contributions of philosophers like Socrates and Aristotle. During the medieval period, ethical thought was strongly influenced by religious teachings. In the modern period, this focus shifted to a more secular approach concerned with moral experience, reasons for acting, and the consequences of actions. An influential development in the 20th century was the emergence of metaethics.

Existentialism is a family of philosophical views and inquiry that explore the human individual's struggle to lead an authentic life despite the apparent absurdity or incomprehensibility of existence. In examining meaning, purpose, and value, existentialist thought often includes concepts such as existential crises, angst, courage, and freedom. Existentialism is associated with several 19th- and 20th-century European philosophers who shared an emphasis on the human subject, despite often profound differences in thought. Among the 19th-century figures now associated with existentialism are philosophers Søren Kierkegaard and Friedrich Nietzsche, as well as novelist Fyodor Dostoevsky, all of whom critiqued rationalism and concerned themselves with the problem of meaning. The word existentialism, however, was not coined until the mid 20th century, during which it became most associated with contemporaneous philosophers Jean-Paul Sartre, Martin Heidegger, Simone de Beauvoir, Karl Jaspers, Gabriel Marcel, Paul Tillich, and more controversially Albert Camus. Many existentialists considered traditional systematic or academic philosophies, in style and content, to be too abstract and removed from concrete human experience. A primary virtue in existentialist thought is authenticity. Existentialism would influence many disciplines outside of philosophy, including theology, drama, art, literature, and psychology. Existentialist philosophy encompasses a range of perspectives, but it shares certain underlying concepts. Among these, a central tenet of existentialism is that personal freedom, individual responsibility, and deliberate choice are essential to the pursuit of self-discovery and the determination of life's meaning.

In philosophy, empiricism is an epistemological view which holds that true knowledge or justification comes either only or primarily from sensory experience and empirical evidence. It is one of several competing views within epistemology, along with rationalism and skepticism. Empiricists argue that empiricism is a more reliable method of finding the truth than relying purely on logical reasoning, because humans have cognitive biases and limitations which lead to errors of judgement. Empiricism emphasizes the central role of empirical evidence in the formation of ideas, rather than innate ideas or traditions. Empiricists may argue that traditions (or customs) arise due to relations of previous sensory experiences. Historically, empiricism was associated with the "blank slate" concept (tabula rasa), according to which the human mind is "blank" at birth and develops its thoughts only through later experience. Empiricism in the philosophy of science emphasizes evidence, especially as discovered in experiments. It is a fundamental part of the scientific method that all hypotheses and theories must be tested against observations of the natural world rather than resting solely on a priori reasoning, intuition, or revelation. Empiricism, often used by natural scientists, holds that "knowledge is based on experience" and that "knowledge is tentative and probabilistic, subject to continued revision and falsification". Empirical research, including experiments and validated measurement tools, guides the scientific method.

Friedrich Wilhelm Nietzsche (15 October 1844 – 25 August 1900) was a German philosopher and writer who started his career as a classical philologist and turned to philosophy early in his academic career. In 1869, at age 24, he was appointed Professor of Classical Philology at the University of Basel. Plagued by health problems for most of his life, he resigned from the university in 1879. He afterward lived as an independent writer, spending much of his life in relative solitude and financial insecurity while moving between Switzerland, Italy, and southern France in search of climates that might alleviate his condition, and in the following decade, he completed much of his core writing. In 1889, aged 44, he suffered a neurological collapse, and thereafter a complete loss of his mental faculties, with paralysis and vascular dementia; he lived out his remaining 11 years, first under the care of his mother, and later his sister, until his death. His works and his philosophy have fostered not only extensive scholarship but also much popular interest. Nietzsche's work encompasses poetry, cultural criticism, and philosophical essays while displaying a fondness for aphorisms and irony. Prominent elements of his philosophy include his critique of objective truth in favour of perspectivism; a genealogical critique of Christian morality and a related theory of master–slave morality; the affirmation of life in response to both the weakening of religion ("God is dead") and the crisis of passive nihilism which followed it; the notion of Apollonian and Dionysian duality; and a characterisation of the human subject as the expression of competing wills, collectively understood as the will-to-power. His later work developed influential concepts such as the Übermensch, the transvaluation of values, and his unique formulation of eternal return. His body of work touched a wide range of topics, including aesthetic, philology, history, music, religion, tragedy, culture and science, and drew inspiration from such diverse sources as Greek tragedy, Arthur Schopenhauer, Ralph Waldo Emerson, and Richard Wagner. After Nietzsche's death, his sister, Elisabeth Förster-Nietzsche, became the curator and editor of his manuscripts. She edited his unpublished writings to fit her German ultranationalist ideology, often contradicting or obfuscating Nietzsche's stated opinions, which were explicitly opposed to antisemitism and nationalism. Through her published editions, Nietzsche's work became associated with fascism and Nazism. Twentieth-century scholars such as Walter Kaufmann, R. J. Hollingdale and Georges Bataille defended Nietzsche against this interpretation, and corrected editions of his writings were soon made available. The works of Nietzsche then achieved renewed popularity in the 1960s and his ideas have since had a widespread impact on 20th- and 21st-century thinkers across philosophy—especially in schools of continental philosophy such as existentialism, postmodernism and post-structuralism—as well as art, literature, music, poetry, politics and popular culture.

Immanuel Kant (born Emanuel Kant; 22 April 1724 – 12 February 1804) was a German philosopher. Born in Königsberg in the Kingdom of Prussia, he is considered one of the central thinkers of the Enlightenment. His comprehensive and systematic works in epistemology, metaphysics, logic, ethics, aesthetics, political theory, and the philosophy of religion have made him one of the most influential and highly discussed figures in modern Western philosophy. Kant's philosophy is centered on the human subject and motivated by the desire to secure the possibility of both knowledge and morality against the threats of skepticism and determinism. In the Critique of Pure Reason (1781/1787), Kant argues for transcendental idealism, the doctrine that space and time are mere "forms of intuition" (German: Anschauung) that structure all experience and that we have knowledge only of "appearances" and not of the nature of things in themselves. Kant drew a parallel to the Copernican Revolution in his proposal to think of the objects of experience as conforming to people's spatial and temporal forms of intuition and the categories of the understanding, instead of the traditional method of showing how the mind might conform to its objects. Kant believed that reason is the source of morality and that the categorical imperative binds all rational agents. He believed that aesthetics arises from a faculty of disinterested judgment. Kant hoped that perpetual peace could be secured through an international federation of republican states and international cooperation. Kant believed that true religion is grounded on morality. The exact nature of his religious views is a matter of dispute.

John Locke (; 29 August 1632 (O.S.) – 28 October 1704 (O.S.)) was an English philosopher and physician, widely regarded as one of the most influential of the Enlightenment thinkers and commonly known as the "father of liberalism". His important works include A Letter Concerning Toleration (1689), Two Treatises of Government (1689/90), both published anonymously, and An Essay Concerning Human Understanding (1689/90). His writing on toleration contends that religion is a matter for the individual and that the churches are voluntary associations, ruling out religious coercion and uniformity; these lead to the idea of separation of church and state. His Two Treatises on Government argues for government based on the consent of the governed and the right to revolt against tyrannous government which has lost consent. The Two Treatises are believed to have influenced the language that Thomas Jefferson chose in his drafting the July 1776 Declaration of Independence during the War of American Independence. Locke lived through the tumultuous political era of the English Civil War and Commonwealth of England after the execution of Charles I, Restoration of the Stuart monarchy, and the 1688 Glorious Revolution. These experiences affected his political thinking and life choices. During the Interregnum, Locke won a place at Christ Church, Oxford after attending the prestigious Westminster School. He spent 15 years at Oxford, first as a student, then as a tutor, pursuing medical and other scientific interests in a circle of friends. In 1666, Locke became an associate of Lord Shaftesbury, a key figure in English political life after the Restoration (1660), and Locke was appointed to governmental posts at Shaftesbury's recommendation. Locke was elected a Fellow of the Royal Society (1668). When Shaftesbury fell from royal favour and died shortly thereafter, Locke went into political exile for five years in the Netherlands (1683–85). There he wrote some of his most important works. During this period, he gained the patronage of Thomas Herbert, 8th Earl of Pembroke. Locke returned to England from exile, accompanying Queen Mary II in 1689. He published three of his most notable works soon after his return. He served as a Commissioner of Trade and Plantations, then retired from public life due to ill health. For the last fourteen years of his life, he lived in the household of Sir Francis Masham and his wife, philosopher Lady Masham, whom Locke had known since she was a young woman. Considered one of the first of the British empiricists, following the tradition of Francis Bacon, Locke is equally important to social contract theory. His work greatly affected the development of epistemology and political philosophy. His writings influenced Voltaire, Jean-Jacques Rousseau, and many Scottish Enlightenment thinkers, as well as the American Revolutionaries. His contributions to classical republicanism and liberal theory are reflected in the United States Declaration of Independence. Internationally, Locke's political-legal principles continue to have a profound influence on the theory and practice of limited representative government and the protection of basic rights and freedoms under the rule of law. Locke's philosophy of mind is often cited as the origin of modern conceptions of personal identity and the psychology of self, figuring prominently in the work of later philosophers, such as Rousseau, David Hume, and Immanuel Kant. He postulated that, at birth, the mind is a blank slate, or tabula rasa. Contrary to Cartesian philosophy based on preexisting concepts, he maintained that we are born without innate ideas, and that knowledge is instead determined only by experience derived from sense perception, a concept now known as empiricism. Locke is often credited for describing private property as a natural right, arguing that when a person—metaphorically—mixes their labour with nature, resources can be removed from the common state of nature.

Jean-Paul Charles Aymard Sartre (, US also ; French: [saʁtʁ]; 21 June 1905 – 15 April 1980) was a French philosopher, playwright, novelist, screenwriter, political activist, biographer, and literary critic, considered a leading figure in 20th-century French philosophy and Marxism. Sartre was one of the key figures in the philosophy of existentialism (and phenomenology). His work has influenced sociology, critical theory, post-colonial theory, and literary studies. He was awarded the 1964 Nobel Prize in Literature despite attempting to refuse it, saying that he always declined official honors and that "a writer should not allow himself to be turned into an institution." Sartre had an open relationship with prominent feminist and fellow existentialist philosopher Simone de Beauvoir. Together, Sartre and de Beauvoir challenged the cultural and social assumptions and expectations of their upbringings, which they considered bourgeois, in both lifestyles and thought. The conflict between oppressive, spiritually destructive conformity (mauvaise foi, literally, 'bad faith') and an "authentic" way of "being" became the dominant theme of Sartre's early work, a theme embodied in his principal philosophical work Being and Nothingness (L'Être et le Néant, 1943). Sartre provided an introduction to his philosophy in his work Existentialism Is a Humanism (L'existentialisme est un humanisme, 1946), originally presented as a lecture. Born in Paris, Sartre lost his father at age two and was raised primarily by his mother and grandfather, who introduced him to literature. He studied at the prestigious École Normale Supérieure, where he developed a deep interest in philosophy, influenced by thinkers like Henri Bergson, Edmund Husserl, and Martin Heidegger. Sartre's early academic career included teaching in several French lycées and engaging in provocative pranks and debates. Sartre's life was marked by strong political engagement. During World War II, he was drafted, captured, and later released, after which he co-founded the resistance group Socialisme et Liberté. Though the group dissolved, Sartre became an influential voice in occupied France, contributing to underground literature and writing plays like No Exit. After the war, he co-founded the journal Les Temps modernes and increasingly used his platform to advocate for political and social causes. He supported anti-colonial movements, condemned French policies in Algeria, opposed U.S. intervention in Vietnam, and aligned himself at various times with Marxism and Maoism. Despite declining health in his later years, Sartre remained committed to activism and intellectual debate until his death in 1980. His funeral drew 50,000 mourners.

Karl Marx (German: [ˈkaʁl ˈmaʁks]; 5 May 1818 – 14 March 1883) was a German philosopher, social and political theorist, economist, journalist, and revolutionary socialist. He developed the theory of historical materialism, analyzing class struggle under capitalism and predicting the system's overthrow by the proletariat in favour of communism. Marx co-authored The Communist Manifesto (1848) with his lifelong friend Friedrich Engels, and undertook a critique of classical political economy in his magnum opus, Das Kapital (1867–1894). Marx's ideas and their subsequent development, collectively known as Marxism, have had enormous influence and have influenced revolutions and uprisings in many countries. Born in Trier in the Kingdom of Prussia, Marx earned a doctoral degree in philosophy from the University of Jena in 1841. A Young Hegelian, he was influenced by the philosophy of Georg Wilhelm Friedrich Hegel, and both critiqued and developed Hegel's ideas in works such as The German Ideology (written 1846) and the Grundrisse (written 1857–1858). While in Paris, Marx wrote his Economic and Philosophic Manuscripts of 1844 and met Engels, who became a close collaborator. After moving to Brussels in 1845, they were active in the Communist League, and in 1848 wrote The Communist Manifesto, which expresses Marx's ideas and lays out a programme for revolution. Marx was expelled from Belgium and Germany, and in 1849 moved to London, where he wrote The Eighteenth Brumaire of Louis Bonaparte (1852) and Das Kapital. From 1864, Marx was involved in the International Workingmen's Association (First International), in which he fought the influence of anarchists led by Mikhail Bakunin. In his Critique of the Gotha Programme (1875), Marx wrote on revolution, the state and the transition to communism. He died stateless in 1883 and was buried in Highgate Cemetery. Marx's critiques of history, society and political economy hold that human societies develop through class conflict. In the capitalist mode of production, this manifests itself in the conflict between the ruling classes (the bourgeoisie) that control the means of production and the working classes (the proletariat) that enable these means by selling their labour power for wages. Employing his historical materialist approach, Marx predicted that capitalism produced internal contradictions like previous socioeconomic systems, and that these tensions would lead to its self-destruction and replacement by a new system. For Marx, class antagonisms under capitalism—owing in part to its instability and crisis-prone nature—would eventuate the working class's development of class consciousness, leading to their conquest of political power and eventually the establishment of a classless, communist society constituted by a free association of producers. Marx actively pressed for its implementation, arguing that the working class should carry out organised proletarian revolutionary action to topple capitalism and bring about socio-economic emancipation. Marx has been described as one of the most influential figures of the modern era, and his work has been both lauded and criticised. Marxism has exerted major influence on socialist thought and political movements, with Marxist schools of thought such as Marxism–Leninism and its offshoots becoming the guiding ideologies of revolutions that took power in many countries during the 20th century, forming communist states. Marx's work in economics has had a strong influence on modern heterodox theories of labour and capital, and he is often cited as one of the principal architects of modern sociology.

Metaphysics is the branch of philosophy that examines the basic structure of reality. It is traditionally seen as the study of mind-independent features of the world, but some theorists view it as an inquiry into the conceptual framework of human understanding. Some philosophers, including Aristotle, designate metaphysics as the first philosophy to suggest that it is more fundamental than other forms of philosophical inquiry. Metaphysics encompasses a wide range of general and abstract topics. It investigates the nature of existence, the features all entities have in common, and their division into categories of being. An influential division is between particulars and universals. Particulars are unique individual entities, like a specific apple. Universals are general features that different particulars have in common, like the color red. Modal metaphysics examines what it means for something to be possible or necessary. Metaphysicians also explore the concepts of space, time, and change, and their connection to causality and the laws of nature. Other topics include how mind and matter are related, whether everything in the world is predetermined, and whether there is free will. Metaphysicians use various methods to conduct their inquiry. They typically rely on rational intuitions and abstract reasoning but also include empirical approaches associated with scientific theories. Due to the abstract nature of its topic, metaphysics has received criticisms questioning the reliability of its methods and the meaningfulness of its theories. Metaphysics is relevant to many fields of inquiry that often implicitly rely on metaphysical concepts and assumptions. The roots of metaphysics lie in antiquity with speculations about the nature and origin of the universe, like those found in the Upanishads in ancient India, Taoism in ancient China, and pre-Socratic philosophy in ancient Greece. During the subsequent medieval period in the West, discussions about the nature of universals were influenced by the philosophies of Plato and Aristotle. The modern period saw the emergence of various comprehensive systems of metaphysics, many of which embraced idealism. In the 20th century, traditional metaphysics in general and idealism in particular faced various criticisms, which prompted new approaches to metaphysical inquiry.

Nihilism is a family of philosophical views that reject the existence of any objectively meaningful purpose, moral value, truth, or knowledge. Nihilistic views span several branches of philosophy, including ethics, value theory, epistemology, and metaphysics. Nihilism is also described as a broad cultural phenomenon or historical movement that pervades modernity in the Western world. Existential nihilism asserts that life has no objective meaning or purpose. The idea that all individual and societal values are ultimately pointless has been associated with various responses. They range from general indifference and existential crises to transformative reinterpretations of established ideals and a creative embrace of personal meaning-making. Moral nihilism, a related view, denies the objective existence of morality, arguing that moral evaluations and practices rest on misguided assumptions without any foundation in external reality. In epistemology or the theory of knowledge, nihilism challenges knowledge and truth. According to relativism, knowledge, truth, or meaning are relative to the perspectives of specific individuals or cultural contexts. This implies there is no independent framework to assess which opinion is ultimately correct. Skeptical interpretations go further by denying the existence of knowledge or truth altogether. In metaphysics, one form of nihilism states that the universe could have been empty without any objects. This view holds that there is no fundamental reason for why something exists rather than nothing. Mereological nihilism asserts there are only simple objects, like elementary particles, but no composite objects, like tables. Cosmological nihilism is the view that reality is unintelligible and indifferent to human understanding. Other nihilist positions include political, semantic, logical, and therapeutic nihilism. Some aspects of nihilism have their roots in ancient philosophy in the form of challenges to established beliefs, values, and practices. However, nihilism is primarily associated with modernity, emerging in the 18th and 19th centuries, particularly in Germany and Russia through the works of Friedrich Heinrich Jacobi and Ivan Turgenev. It took center stage in the thought of Friedrich Nietzsche, who understood nihilism as a pervasive cultural trend in which people lose the traditional values and ideals guiding their lives as a result of secularization. In the 20th century, nihilist themes were explored by Dadaism, existentialism, and postmodern philosophy.

Plato ( PLAY-toh; Ancient Greek: Πλάτων, Plátōn; born c. 428–423 BC, died 348/347 BC) was an ancient Greek philosopher of Classical Athens who is most commonly considered the foundational thinker of the Western philosophical tradition. An innovator of the literary dialogue and dialectic forms, Plato influenced all the major areas of theoretical philosophy and practical philosophy, and was the founder of the Academy, a philosophical school in Athens where Plato taught the collection of philosophical theories that would later become known as Platonism. Plato's most famous contribution is his Theory of Forms (or Ideas), which aims to solve what is now known as the problem of universals. He was influenced by the pre-Socratic thinkers Pythagoras, Heraclitus, and Parmenides, although much of what is known about them is derived from Plato himself. Along with his teacher Socrates, and his student Aristotle, Plato is a central figure in the history of Western philosophy. Plato's complete works are believed to have survived for over 2,400 years—unlike that of nearly all of his contemporaries. Although their popularity has fluctuated, they have consistently been read and studied through the ages. Through Platonism's outgrowth Neoplatonism, he also influenced Christian, Jewish and Islamic philosophy. In modern times, Alfred North Whitehead said: "the safest general characterization of the European philosophical tradition is that it consists of a series of footnotes to Plato."

Free will is generally understood as the capacity or ability of people to (a) choose between different possible courses of action, (b) exercise control over their actions in a way that is necessary for moral responsibility, or (c) be the ultimate source or originator of their actions. There are different theories as to its nature, and these aspects are often emphasized differently depending on philosophical tradition, with debates focusing on whether and how such freedom can coexist with physical determinism, divine foreknowledge, and other constraints. Free will is closely linked to the concepts of moral responsibility and moral desert, praise, culpability, and other judgements that can logically apply only to actions that are freely chosen. It is also connected with the concepts of advice, persuasion, deliberation, and prohibition. Traditionally, only actions that are freely willed are seen as deserving credit or blame. Whether free will exists and the implications of whether it exists or not constitute some of the longest-running debates of philosophy. Some philosophers and thinkers conceive of free will as the capacity to make choices undetermined by past events. However, determinism suggests that the natural world is governed by cause-and-effect relationships, and only one course of events is possible, which is inconsistent with a libertarian model of free will. Ancient Greek philosophy identified this issue, which remains a major focus of philosophical debate to this day. The view that posits free will as incompatible with determinism is called incompatibilism and encompasses both metaphysical libertarianism (the claim that determinism is false and thus free will is at least possible) and hard determinism or hard incompatibilism (the claim that determinism is true and thus free will is not possible). Another incompatibilist position is illusionism or hard incompatibilism, which holds not only determinism but also indeterminism (randomness) to be incompatible with free will and thus free will to be impossible regardless of the metaphysical truth of determinism. In contrast, compatibilists hold that free will is compatible with determinism. Some compatibilist philosophers (i.e., hard compatibilists) even hold that determinism is actually necessary for the existence of free will and agency, on the grounds that choice involves preference for one course of action over another, requiring a sense of how choices will turn out. Different compatibilists offer very different definitions of what "free will" means and consequently find different types of constraints to be relevant to the issue. Classical compatibilists considered free will nothing more than freedom of action, considering one free of will if, had one counterfactually wanted to do otherwise, one could have done otherwise without physical impediment. Many contemporary compatibilists instead identify free will as a psychological capacity, such as to direct one's behavior in a way that is responsive to reason or potentially sanctionable. There are still further different conceptions of free will, each with its own concerns, sharing only the common feature of not finding the possibility of physical determinism a threat to the possibility of free will.

Phenomenology is a philosophical study and movement largely associated with the early 20th century that seeks to objectively investigate the nature of subjective, conscious experience. It attempts to describe the universal features of consciousness while avoiding assumptions about the external world, aiming to describe phenomena as they appear, and to explore the meaning and significance of lived experience. This approach, while philosophical, has found many applications in qualitative research across different scientific disciplines, especially in the social sciences, humanities, psychology, and cognitive science, but also in fields as diverse as health sciences, architecture, and human-computer interaction, among many others. The application of phenomenology in these fields aims to gain a deeper understanding of subjective experience, rather than focusing on behavior. Phenomenology is contrasted with phenomenalism, which reduces mental states and physical objects to complexes of sensations, and with psychologism, which treats logical truths or epistemological principles as the products of human psychology. In particular, transcendental phenomenology, as outlined by Edmund Husserl, aims to arrive at an objective understanding of the world via the discovery of universal logical structures in human subjective experience. There are important differences in the ways that different branches of phenomenology approach subjectivity. For example, according to Martin Heidegger, truths are contextually situated and dependent on the historical, cultural, and social context in which they emerge. Other types include hermeneutic, genetic, and embodied phenomenology. All these different branches of phenomenology may be seen as representing different philosophies despite sharing the common foundational approach of phenomenological inquiry; that is, investigating things just as they appear, independent of any particular theoretical framework.

Pragmatism is a philosophical tradition that views language and thought as tools for prediction, problem solving, and action, rather than describing, representing, or mirroring reality. Pragmatists contend that most philosophical topics—such as the nature of knowledge, language, concepts, meaning, belief, and science—are best viewed in terms of their practical uses and successes. Pragmatism began in the United States in the 1870s. Its origins are often attributed to philosophers Charles Sanders Peirce, William James and John Dewey. In 1878, Peirce described it in his pragmatic maxim: "Consider the practical effects of the objects of your conception. Then, your conception of those effects is the whole of your conception of the object."

Philosophy of mind is a branch of philosophy that deals with the nature of the mind and its relation to the body and the external world. The mind–body problem is a paradigmatic issue in philosophy of mind, although a number of other issues are addressed, such as the hard problem of consciousness and the nature of particular mental states. Aspects of the mind that are studied include mental events, mental functions, mental properties, consciousness and its neural correlates, the ontology of the mind, the nature of cognition and of thought, and the relationship of the mind to the body. Dualism and monism are the two central schools of thought on the mind–body problem, although nuanced views have arisen that do not fit either category neatly. Dualism finds its entry into Western philosophy thanks to René Descartes in the 17th century. Substance dualists like Descartes argue that the mind is an independently existing substance, whereas property dualists maintain that the mind is a group of independent properties that emerge from and cannot be reduced to the brain, but that it is not a distinct substance. Monism is the position that mind and body are ontologically indiscernible entities, not dependent substances. This view was espoused by the 17th-century rationalist Baruch Spinoza. Physicalists argue that only entities postulated by physical theory exist, and that mental processes will eventually be explained in terms of these entities as physical theory continues to evolve. Physicalists maintain various positions on the prospects of reducing mental properties to physical properties (many of whom adopt compatible forms of property dualism), and the ontological status of such mental properties remains unclear. Idealists maintain that the mind is all that exists and that the external world is either mental itself, or an illusion created by the mind. Neutral monists such as Ernst Mach and William James argue that events in the world can be thought of as either mental (psychological) or physical depending on the network of relationships into which they enter, and dual-aspect monists such as Spinoza adhere to the position that there is some other, neutral substance, and that both matter and mind are properties of this unknown substance. The most common monisms in the 20th and 21st centuries have all been variations of physicalism; these positions include behaviorism, type physicalism, anomalous monism and functionalism. Most modern philosophers of mind adopt either a reductive physicalist or non-reductive physicalist position, maintaining in their different ways that the mind is not something separate from the body. These approaches have been particularly influential in the sciences, especially in the fields of sociobiology, computer science (specifically, artificial intelligence), evolutionary psychology and the various neurosciences. Reductive physicalists assert that all mental states and properties will eventually be explained by scientific accounts of physiological processes and states. Non-reductive physicalists argue that although the mind is not a separate substance, mental properties supervene on physical properties, or that the predicates and vocabulary used in mental descriptions and explanations are indispensable, and cannot be reduced to the language and lower-level explanations of physical science. Continued neuroscientific progress has helped to clarify some of these issues; however, they are far from being resolved. Modern philosophers of mind continue to ask how the subjective qualities and the intentionality of mental states and properties can be explained in naturalistic terms.

Logic is the study of correct reasoning. It includes both formal and informal logic. Formal logic is the study of deductively valid inferences or logical truths. It examines how conclusions follow from premises based on the structure of arguments alone, independent of their topic and content. Informal logic is associated with informal fallacies, critical thinking, and argumentation theory. Informal logic examines arguments expressed in natural language whereas formal logic uses formal language. When used as a countable noun, the term "a logic" refers to a specific logical formal system that articulates a proof system. Logic plays a central role in many fields, such as philosophy, mathematics, computer science, and linguistics. Logic studies arguments, which consist of a set of premises that leads to a conclusion. An example is the argument from the premises "it's Sunday" and "if it's Sunday then I don't have to work" leading to the conclusion "I don't have to work." Premises and conclusions express propositions or claims that can be true or false. An important feature of propositions is their internal structure. For example, complex propositions are made up of simpler propositions linked by logical vocabulary like ∧ {\displaystyle \land } (and) or → {\displaystyle \to } (if...then). Simple propositions also have parts, like "Sunday" or "work" in the example. The truth of a proposition usually depends on the meanings of all of its parts. However, this is not the case for logically true propositions. They are true only because of their logical structure independent of the specific meanings of the individual parts. Arguments can be either correct or incorrect. An argument is correct if its premises support its conclusion. Deductive arguments have the strongest form of support: if their premises are true then their conclusion must also be true. This is not the case for ampliative arguments, which arrive at genuinely new information not found in the premises. Many arguments in everyday discourse and the sciences are ampliative arguments. They are divided into inductive and abductive arguments. Inductive arguments are statistical generalizations, such as inferring that all ravens are black based on many individual observations of black ravens. Abductive arguments are inferences to the best explanation, for example, when a doctor concludes that a patient has a certain disease which explains the symptoms they suffer. Arguments that fall short of the standards of correct reasoning often embody fallacies. Systems of logic are theoretical frameworks for assessing the correctness of arguments. Logic has been studied since antiquity. Early approaches include Aristotelian logic, Stoic logic, Nyaya, and Mohism. Aristotelian logic focuses on reasoning in the form of syllogisms. It was considered the main system of logic in the Western world until it was replaced by modern formal logic, which has its roots in the work of late 19th-century mathematicians such as Gottlob Frege. Today, the most commonly used system is classical logic. It consists of propositional logic and first-order logic. Propositional logic only considers logical relations between full propositions. First-order logic also takes the internal parts of propositions into account, like predicates and quantifiers. Extended logics accept the basic intuitions behind classical logic and apply it to other fields, such as metaphysics, ethics, and epistemology, as frameworks for reasoning about what is possible or necessary, what is or should be, and what is believed or known. Deviant logics, on the other hand, reject certain classical intuitions and provide alternative explanations of the basic laws of logic.

Baruch (de) Spinoza (24 November 1632 – 21 February 1677), also known under his Latinized pen name Benedictus de Spinoza, was a philosopher of Portuguese-Jewish origin, who was born and lived in the Dutch Republic. A forerunner of the Enlightenment, Spinoza significantly influenced modern biblical criticism, 17th-century rationalism, and Dutch intellectual culture, establishing himself as one of the most important and radical philosophers of the early modern period. Influenced by Stoicism, Thomas Hobbes, René Descartes, and heterodox Christians, Spinoza was a leading philosopher of the Dutch Golden Age. Spinoza was born in Amsterdam to a Marrano family that fled Portugal for the more tolerant Dutch Republic. He received a traditional Jewish education, learning Hebrew and studying sacred texts within the Portuguese Jewish community, where his father was a prominent merchant. As a young man, Spinoza challenged rabbinic authority and questioned Jewish doctrines, leading to his permanent expulsion from his Jewish community in 1656. Following that expulsion, he distanced himself from all religious affiliations and devoted himself to philosophical inquiry, supporting himself by lens grinding. Spinoza attracted a dedicated circle of followers who gathered to discuss his writings and joined him in the intellectual pursuit of truth. Spinoza published sparingly during his lifetime to circumvent persecution and the suppression of his works. In his Tractatus Theologico-Politicus, described by Steven Nadler as "one of the most important books of Western thought", Spinoza questioned the divine origin of the Hebrew Bible and the nature of God while arguing that ecclesiastic authority should have no role in a secular, democratic state. Ethics equates God with nature, arguing for a view of God that scholars interpret variously as pantheistic, panentheistic, or atheistic, and it explores the place of human freedom in a world devoid of theological, cosmological, and political moorings. Rejecting messianism and the emphasis on the afterlife, Spinoza emphasized appreciating and valuing life for oneself and others. By advocating for individual liberty in its moral, psychological, and metaphysical dimensions, Spinoza helped establish the genre of political writing called secular theology. Spinoza’s philosophy addresses multiple areas of philosophical discourse, including metaphysics, epistemology, political philosophy, ethics, philosophy of mind, and philosophy of science. His works were published posthumously by his friends and influenced philosophers for the next two centuries. Spinoza is celebrated as one of the most original and influential thinkers of the seventeenth century. Rebecca Goldstein described him as "the renegade Jew who gave us modernity".

David Hume (; born David Home; 7 May 1711 – 25 August 1776) was a Scottish philosopher, historian, economist and essayist who is known for his highly influential system of empiricism, philosophical scepticism and metaphysical naturalism. Beginning with A Treatise of Human Nature (1739–40), Hume strove to create a naturalistic science of man that examined the psychological basis of human nature. Hume followed John Locke in rejecting the existence of innate ideas, concluding that all human knowledge derives solely from experience; this places him amongst such empiricists as Francis Bacon, Thomas Hobbes, Locke and George Berkeley. Hume argued that inductive reasoning and belief in causality cannot be justified empirically; instead, they result from custom and mental habit. People never actually perceive that one event causes another but experience only the "constant conjunction" of events. This problem of induction means that to draw any causal inferences from past experience, it is necessary to presuppose that the future will resemble the past; this metaphysical presupposition cannot itself be grounded in prior experience. An opponent of philosophical rationalists, Hume held that passions rather than reason govern human behaviour, proclaiming that "Reason is, and ought only to be the slave of the passions." Hume was also a sentimentalist who held that ethics are based on emotion or sentiment rather than abstract moral principle. He maintained an early commitment to naturalistic explanations of moral phenomena and is usually accepted by historians of European philosophy to have first clearly expounded the is–ought problem, or the idea that a statement of fact alone can never give rise to a normative conclusion of what ought to be done. Hume denied that people have an actual conception of the self, positing that they experience only a bundle of sensations and that the self is nothing more than this bundle of perceptions connected by an association of ideas. Hume's compatibilist theory of free will takes causal determinism as fully compatible with human freedom. His philosophy of religion, including his rejection of miracles and critique of the argument from design, was especially controversial. Hume left a legacy that affected utilitarianism, logical positivism, the philosophy of science, early analytic philosophy, cognitive science, theology and many other fields and thinkers. Immanuel Kant credited Hume as the inspiration that had awakened him from his "dogmatic slumbers".

John Stuart Mill (20 May 1806 – 7 May 1873) was an English philosopher, political economist, politician and civil servant. One of the most influential thinkers in the history of liberalism and social liberalism, he contributed widely to social theory, political theory, and political economy. Dubbed "the most influential English-speaking philosopher of the nineteenth century" by the Stanford Encyclopedia of Philosophy, he conceived of liberty as justifying the freedom of the individual in opposition to unlimited state and social control. He advocated political and social reforms such as proportional representation, the emancipation of women, and the development of labour organisations and farm cooperatives. The Columbia Encyclopedia describes Mill as occasionally coming "close to socialism, a theory repugnant to his predecessors". He was a proponent of utilitarianism, an ethical theory developed by his predecessor Jeremy Bentham. He contributed to the investigation of scientific methodology, though his knowledge of the topic was based on the writings of others, notably William Whewell, John Herschel, and Auguste Comte, and research carried out for Mill by Alexander Bain. He engaged in a written debate with Whewell. A member of the Liberal Party and co-author of the early feminist work The Subjection of Women (1869), Mill was also the second Member of Parliament to call for women's suffrage after Henry Hunt in 1832. The ideas presented in his essay On Liberty (1859) have remained the basis of much political thought, and a copy is passed to the president of the Liberal Democrats (the successor party to Mill's own) as a symbol of office.

Thomas Hobbes ( HOBZ; 5 April 1588 – 4 December 1679) was an English philosopher and political theorist, best known for his 1651 book Leviathan, in which he expounds an influential formulation of social contract theory. He is considered to be one of the founders of modern political philosophy. In his early life, overshadowed by his father's departure following a fight, he was taken under the care of his wealthy uncle. Hobbes's academic journey began in Westport, leading him to the University of Oxford, where he was exposed to classical literature and mathematics. He then graduated from the University of Cambridge in 1608. He became a tutor to the Cavendish family, which connected him to intellectual circles and initiated his extensive travels across Europe. These experiences, including meetings with figures like Galileo, shaped his intellectual development. After returning to England from France in 1637, Hobbes witnessed the destruction and brutality of the English Civil War from 1642 to 1651 between Parliamentarians and Royalists, which heavily influenced his advocacy for governance by an absolute sovereign in Leviathan, as the solution to human conflict and societal breakdown. Aside from social contract theory, Leviathan also popularised ideas such as the state of nature ("war of all against all") and laws of nature. His other major works include the trilogy De Cive (1642), De Corpore (1655), and De Homine (1658) as well as the posthumous work Behemoth (1681). Hobbes contributed to a diverse array of fields, including history, jurisprudence, geometry, optics, theology, classical translations, ethics, as well as philosophy in general, marking him as a polymath. Despite controversies and challenges to his reputation, including accusations of atheism and contentious debates with contemporaries, Hobbes's work profoundly influenced the understanding of political structure and human nature.

Simone Lucie Ernestine Marie Bertrand de Beauvoir (9 January 1908 – 14 April 1986) was a French existentialist philosopher, writer, social theorist, and feminist activist. Though she did not consider herself a philosopher, nor was she considered one at the time of her death, she had a significant influence on both feminist existentialism and feminist theory. Beauvoir wrote novels, essays, short stories, biographies, autobiographies, and monographs on philosophy, politics, and social issues. She was best known for her "trailblazing work in feminist philosophy", The Second Sex (1949), a detailed analysis of women's oppression and a foundational tract of contemporary feminism. She was also known for her novels, the most famous of which were She Came to Stay (1943) and The Mandarins (1954). Her most enduring contribution to literature is her memoirs, notably the first volume, Mémoires d'une jeune fille rangée (1958). She received the 1954 Prix Goncourt, the 1975 Jerusalem Prize, and the 1978 Austrian State Prize for European Literature. She was also nominated for the Nobel Prize in Literature in 1961, 1969 and 1973. However, Beauvoir generated controversy when she briefly lost her teaching job after being accused of sexually abusing some of her students.