6th century BC: Thales of Miletus (Greek): efforts to explain natural phenomena without reference to mythology.
5th century BC: Zeno of Elea (Greek): best known for his profound paradoxes.
5th century BC: Socrates (Greek): the Socratic Dialogues which are a series of dialogues written by Plato in the form of discussions between Socrates and other persons of his time illustrating the Socratic method (a form of debate between individuals with opposing viewpoints based on asking and answering questions to stimulate critical thinking). Socrates is known chiefly through the writings of his student Plato and it is a matter of debate which Socrates Plato is describing - the historical figure or Plato's fictionalization.
4th-5th century BC: Plato (Greek): Theory of Forms (Ideas): non-material abstract forms, and not the material world known to us through sensation, possess the highest kind of reality.
4th century BC: Aristotle (Greek) - a student of Plato and teacher of Alexander the Great. Aristotle's writings were the first to create a comprehensive system of Western philosophy, encompassing morality, aesthetics, logic, science, politics and metaphysics.
1274: Thomas Aquinas (Italian): in his Summa Theologica he elaborates on the existence of God, Creation, Man's purpose, Christ, the Sacraments and back to God.
1637: Rene Descartes (French): best known for his Cogito ergo sum (I think, therefore I am) - the phrase became a fundamental element of Western philosophy, as it was perceived to form a foundation for all knowledge.
1651: Thomas Hobbes (English): in his book Leviathan he established the foundation for Western political philosophy from the perspective of social contract theory (the appropriate relationships between individuals and their governments).
1677: Baruch Spinoza (Dutch-Jewish): identified God with Nature (Ethics).
1688: John Locke (English): his liberal theory is reflected in the American Declaration of Independence (Second treatise of government).
1710: George Berkeley (Anglo-Irish): immaterialism (or subjective idealism). This theory denies the existence of material substance and instead contends that familiar objects are only ideas in the minds of perceivers, and as a result objects cannot exist without being perceived. Thus, as Berkeley famously put it, for physical objects esse est percipi (to be is to be perceived) (Principles of Human Knowledge).
1733: Voltaire (French): advocacy of civil liberties, including freedom of religion, freedom of expression, free trade and separation of church and state (Philosophical Letters).
1739: David Hume (Scottish): philosophical empiricism (knowledge comes primarily via sensory experience) and skepticism (A Treatise of Human Nature).
1755: Jean-Jacques Rousseau (French): influenced the French Revolution as well as the overall development of modern political, sociological and educational thought (Discourse on Political Economy).
1776: Adam Smith (Scottish): divided moral philosophy into four parts: ethics and virtue, private rights and natural liberty, familial rights (economics), and state and individual rights (politics) (The Wealth of Nations).
1781: Immanuel Kant (German): time and space are not materially real but merely the ideal a priori condition of our internal intuition or mind. (Critique of Pure Reason). Idealism philosophy assert that reality is mentally constructed.
1811: Georg Wilhelm Friedrich Hegel (German): one of the creators of German Idealism and an important precursor to Marxism (Science of Logic).
1813: Arthur Schopenhauer (German): known for his pessimism and philosophical clarity (On the Fourfold Root of the Principle of Sufficient Reason which examines the four separate manifestations of reason in the phenomenal world).
1843: Soren Aabye Kierkegaard (Danish): widely considered to be the first existentialist (Two Upbuilding Discourses). In existentialism the individual is solely responsible for giving his life meaning in spite of many existential obstacles and distractions including despair, angst, absurdity, alienation, and boredom.
1848: Karl Marx (German): the chief theorist of modern socialism and communism (The Communist Manifesto and Capital (1867–1894).
1883: Friedrich Nietzsche (German): radical questioning of the value and objectivity of truth (Thus Spoke Zarathustra).
1913: Bertrand Russell * (British) one of the founders of analytic philosophy (characterized by an emphasis on clarity and formal logic and analysis of language and a respect for the natural sciences (Theory of Knowledge).
1927: Martin Heidegger (German): known for his existential explorations of the "question of Being" (Being and Time).
1938: Jean-Paul Sartre * (French): one of the leading figures in 20th century French philosophy, Marxism, and existentialism (Nausea).
1951: Albert Camus * (French-Algerian): contributed to the rise of the philosophy known as absurdism (the conflict between the human tendency to seek value and meaning in life and the human inability to find any) (The Rebel).
1967: Jacques Derrida (French): deconstructionism (an effort to understand a text through its relationships to various contexts since there is nothing outside the text and all of the references used to interpret a text are themselves texts) (Of Grammatology).
* Nobel laureates
Philosophers and Thinkers Biographies
Political Theorists and Activists - Blupete
Bjorns Guide to Philosophy
Biographies of Famous Philosophers - about.com
The Internet Encyclopedia of Philosophy
Major Figures in Western Philosophy - Garth Kemerling
Stanford Encyclopedia of Philosophy
History of Philosophy - friesian.com
Philosophers - EpistemeLinks.com
Guide to Philosophy on the Internet - Peter Suber, Earlham College
Biographies and Writings of Well Known Anarchists
Individual Philosophers - Peter J. King, Pembroke College, Oxford
Evolution of Western Thinking About Nature
A Timeline of Western Philosophers - Garth Kemerling
Philosophers of Science
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