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.

Descartes today: Cartesian dualism is neatly explained by the film The Matrix. In it, the protagonist, Neo, is faced with a classic Cartesian mind-versus-body problem. Neo thinks he is working at a software company, but when he is disconnected from the virtual-reality world of the Matrix, he realizes that his life has been a fiction that existed only in his mind. Neo employs firearms and a hovercraft to solve his dualistic dilemma. Descartes would probably have just mulled it over.

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.

Hume today: Hume believed that people become so influenced by events they experience in their lives that they readily jump to the conclusion that history will repeat itself. A girl who has been cheated on by her past three boyfriends will assume that her boyfriend is up to no good when she catches him texting her best friend. However, it’s possible he’s just planning a party for his sweetheart.