Does a reference to Descartes go right over your head? Use this cheat sheet to better understand the big ideas of some of history’s greatest thinkers.

By Mark Adams
Updated August 06, 2009
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Illustration of Rene Descartes' quote "I think therefore I am"
Credit: Heads of State

Plato (428/427–348/347 B.C.)

“The only real ill-doing is the deprivation of knowledge.”

Best-known work: The Republic.

Big ideas: Theory of Forms; platonic relationship.

  • Everything on earth, whether an object (such as a car) or an idea (such as justice), is actually an imperfect copy of an ideal and permanent “form” that exists somewhere, beyond our universe. This is known as the Theory of Forms. The place where all these ideal forms exist is guided by a heavenly force that Plato believed should influence our behavior. (This notion shaped Christianity.) The ideal that was the most important to Plato was moral goodness, which he called “the good.” He believed that we should spend our lives trying to attain absolute goodness, even if we always fall short, because it is the path to happiness.
  • Plato believed that the ideal version of love is a meeting of the minds and doesn’t entail a physical aspect―hence the term “platonic relationship.”

Aristotle (384–322 B.C.)

“Anything that we have to learn to do, we learn by the actual doing of it.”

Best-known work: Nicomachean Ethics (supposedly named after Aristotle’s son, Nicomachus).

Big ideas: Deductive reasoning; golden mean; catharsis.

  • When a person truly understands a topic, she can create a deductive argument―one that starts with a general concept and works toward a more specific one. Aristotle favored a type of deductive reasoning called syllogism (also a favorite of Sherlock Holmes), in which two premises are combined to reach a conclusion: All men are mortal. George Clooney is a man. Therefore George Clooney is mortal. (Although that might be hard to believe.)
  • Life should be lived according to the “golden mean”―what Aristotle called the virtuous halfway point between two vices. For example, courage is the mean between cowardice and rashness.
  • The emotional cleansing one experiences while watching a dramatic performance is what Aristotle termed “catharsis.” For example, you might have had a cathartic moment, with mixed feelings of hope and despair, when Kate Winslet delivered the line “I’ll never let you go, I promise” to a dying Leonardo DiCaprio in Titanic (if you weren’t busy looking at your watch).

René Descartes (1596–1650)

“Cogito, ergo sum.” (“I think, therefore I am.”)

Best-known work: Meditations on First Philosophy.

Big idea: Cartesian dualism.

  • Descartes believed that the mind and its thoughts were not part of the body, or even the physical world. (Although he did believe it communicated with the body through the brain.) This interaction between the mind, which is a nonphysical thing, and the body, which is a physical thing, is known as Cartesian dualism.
  • Proof of one’s existence is not to be found in the three-dimensional world (by tapping one’s forehead, for example) but in the very fact that one is pondering that existence: You think, therefore you are.

David Hume (1711–1776)

“A wise man...proportions his belief to the evidence.”

Best-known work: A Treatise on Human Nature.

Big ideas: Skepticism; empiricism; causation.

  • It is impossible to know anything with complete certainty, outside of the simplest mathematical proofs, according to the skeptical Hume.
  • Everything we think we know comes from our experiences, senses, and habits―that’s the theory of empiricism. For example, once we have seen a glass fall from a table and break, we expect future falling glasses to smash as well.
  • Hume rejected the reasoning that events that occur one after another are a result of cause and effect and will continue to occur in the same way. In other words, that falling glass might break, but that knowledge is not absolute: It probably will, but you can’t be sure.

Immanuel Kant (1724–1804)

“God does not simply will that we should be happy, but rather that we should make ourselves happy.”

Best-known work: Critique of Pure Reason.

Big ideas: Interpretation; transcendental idealism; categorical imperative.

  • No matter how closely we can perceive something (this one-inch cube of ice in my hand is cold, hard, odorless, opaque), we can never fully understand the “thing-in-itself,” or the essence of what something truly is. Our perceptions are just interpretations of the real thing. We can study the way the world appears, but we can’t have knowledge of the world in and of itself. This concept is known as transcendental idealism.
  • When making a moral decision, a person should consider what would happen if everyone took that course of action. For instance, if everyone lied, no one could ever be trusted. So should you lie? No. Kant called this ethical test the “categorical imperative”; he believed that applying it to every action, even if it goes against one’s self-interest, and following the set of rules it implies (don’t lie, cheat, steal, etc.), is the key to leading a righteous life.

G.W.F. Hegel (1770–1831)

“Nothing great has been and nothing great can be accomplished without passion.”

Best-known work: The Phenomenology of Spirit (a tongue twister that basically means the study of consciousness and experience).

Big ideas: Absolute spirit; Zeitgeist; Hegelian dialectic.

  • The universe is a giant network connecting everything. Therefore, every person, object, or idea that has ever existed is part of a greater whole, known as the Absolute spirit.
  • People’s thoughts are guided by the political and cultural atmosphere of a particular moment in history, or what Hegel called the Zeitgeist (which translates from German as time-spirit).
  • Great strides have been made politically and socially through Hegel’s interpretation of the dialectic―a method of argument, first used by the Greek philosophers, based on the theory that a consensus can be reached through the discussion of two dissenting opinions. In Hegel’s view, a thesis is proposed; it is countered by an anti-thesis. Then the two ideas meet in a violent clash and are eventually resolved in a synthesis. This synthesis becomes the new thesis, and the process continues until truth is reached. Revolutionary figures of the 19th and 20th centuries, such as Karl Marx, used Hegel’s formula to influence their arguments.

Back to School

If you are interested in revisiting or learning about some of philosophy’s greatest thinkers, check out Real Simple’s reading list of introductory philosophy books.

  • Heidegger and a Hippo Walk Through Those Pearly Gates: Using Philosophy (and Jokes!) to Explore Life, Death, the Afterlife, and Everything in Between, by Thomas Cathcart and Daniel Klein (Viking, $20,
  • Plato and a Platypus Walk Into a Bar...: Understanding Philosophy Through Jokes, by Thomas Cathcart and Daniel Klein (Penguin, $12,
  • The Consolations of Philosophy, by Alain de Botton (Vintage, $15,
  • Story of Philosophy, by Will Durant (Pocket, $8,
  • What Would Socrates Say?: Philosophers Answer Your Questions About Love, Nothingness, and Everything Else, by Alexander George (Clarkson Potter, $20,
  • A Beginner’s Guide to Philosophy, by Dominique Janicaud (Pegasus, $19,
  • Philosophy Made Simple, by Richard H. Popkin and Avrum Stroll (Butterworth-Heinemann, $40,
  • Philosophy for Beginners, by Richard Osborne and Ralph Edney (Writers and Readers, $15,