RENÉ DESCARTES (1596-1650)

  

“If I think, then I am,” said Descartes,

“It’s an argument dear to my heart.

With dogged persistence

I proved my existence,

That is why the world thinks I'm so smart."

Note: "I think, therefore I am” is probably the most famous philosophical statement ever made.  Descartes states it in French in his Discourse on Method (1637) and then in Latin (“Cogito, ergo sum”) in his Principles of Philosophy (1644), though not (at least, not verbatim) in his equally famous Meditations on First Philosophy (1641).  Much ink has been spilled on whether the “cogito”, as it has come to be known, is really a piece of reasoning, and whether it can provide Descartes with certain knowledge of his own existence, assuming that he has found good reasons to doubt all of his former beliefs (as part of his project to discover an Archimedean point, a foundation on the basis of which the rest of what is certainly knowable can be safely derived).  In any event, Descartes uses both the proposition that he is thinking (which he takes to be certain) and the proposition that he exists (which he derives from the proposition that he is thinking) to prove the existence of a perfect God, a God who is not a deceiver, but who would be deceiving us if the external world that we believe exists were not really there.  From this, Descartes concludes that there really must be an external world.  As a piece of a priori philosophical reasoning for the existence of a world outside our minds, this is a tour de force, even if (as most philosophers believe, myself included) it ultimately fails.