Philosophy Quotations Explained: "I think, therefore I am"

"The proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind."
- Rene Descartes, Meditations on First Philosophy, II.

"Ego cogito, ergo sum..."
- Rene Descartes, Principles of Philosophy, Part 1, Article 7

"I think, therefore I am," is perhaps the most famous quote from all of modern philosophy. When the quote is taken out of context, however, it may seem that Descartes is magically thinking himself into existence, or that he's making some snooty statement about the philosopher's raison d'etre - that the meaning of life is to think.

This latter misinterpretation is spread further by witty slogans on t-shirts and bumper stickers advertising all sorts of hobbies - e.g., "I ski, therefore I am."

But Descartes wasn't talking about how much he liked thinking. He was actually trying to establish the foundations of knowledge - what we can know with certainty.

In his Meditations on First Philosophy, Descartes's project is to find some fundamental proposition that is beyond doubt, and then use it as a starting point from which to derive everything that we know. Descartes writes:

All that I have, up to this moment, accepted as possessed of the highest truth and certainty, I received either from or through the senses. I observed, however, that these sometimes misled us; and it is the part of prudence not to place absolute confidence in that by which we have even once been deceived. (Meditations, I.)

Descartes goes on to point out that everything we perceive is subject to doubt, because we could be dreaming, or our thoughts could even be subject to the manipulations of an evil demon (think a 17th-century version of the Matrix).

So given that our senses always can be deceiving us, can we know anything with certainty? Yes, Descartes says: we know, with certainty, that we are thinking. Even if we are dreaming - even if we're trapped in the Matrix and don't know it - we at least know that we are thinking. And because we know we are thinking, we know that we - in some manner or form - exist. I think, therefore I am. Cogito ergo sum.

Philosophers since his day have split a lot of epistemological hairs over the Cogito and whether it actually proves what Descartes wants it to prove. But his radical project of doubting everything, and applying mathematical logic to problems of knowledge and reality, still makes Descartes the father of modern philosophy.