Cogito ergo sum
Cogito = I think/ am thinking
Ergo = Therefore
Sum =I am / I exist
Cogito ergo sum is the famous phrase ascribed to the French philosopher Rene Descartes. However, the phrase doesn't appear in his writings at all. In his Meditations on First Philosophy Descartes, writing in Latin, makes the claim "sum res cogitans" (I am a thinking being). And in his Discourse on Method, he states in French "Je pense donc je suis", which means "I am thinking therefore I am". However the famous phrase itself was never issued from Descartes' pen. Nevertheless, philosophers have taken to calling the claim "I think therefore I am" the cogito.
Linguistic note: In French and many other languages, there is no difference between saying "I walk" and "I am walking" etc, even though there are two distinct meanings; you walk occasionally, or you are walking now. This is why the cogito is often (probably wrongly) rendered as " I think therefore I am". It makes a lot more sense as "I am thinking therefore I am", as will be seen later.
In order to get an idea of the importance attached to this statement, it helps to understand Descartes' project. His ideas sprang up towards the end of the Renaissance in Europe. For nearly fifteen hundred years, some knowledge went unchallenged. Galen's medical textbooks, Archemedes's laws of motion and Aristotle's scientific musings were taken as fact. If what you saw didn't agree with these authorities, you were probably wrong.
The Renaissance saw the end of all that. For the first time, the authority of these ancient masters was challenged.
Rene Descartes was a mathematician and scientist, but wracked with doubt, doubt about everything. He saw that commonly accepted truths were torn down and replaced with new truths. He wanted to be able to know that these new truths were any firmer than what went before.
So Descartes decided to pull down the whole house and reconstruct it on firmer foundations. He developed what is called the Doubt, where he took everything he believed to be true and asked himself if he could be sure it was true. He allowed the possibility that he was dreaming, or drugged, or a brain in a vat. He allowed the possibility that a malicious demon was fooling him into believing things like 1+1=2, when in fact it was all nonsense. In the end, he decided there was nothing he could be sure of.
Nothing? Well, almost nothing. From the brink, Descartes decides "I am thinking therefore I am", the cogito.
What work is the cogito doing? What is it for?
This is not necessarily simple. There are a number of ways of interpreting the cogito, all giving it a slightly different meaning.
1.An enthymeme, or argument with a hidden premise. Here's one way to build it:
1. I am a thinking thing (I am thinking, sum res cogitans)
2. All thinking things are existent things (hidden premise)
3 I am an existent thing (I am).
The problem with this version is that Descartes isn't allowing himself to use hidden rules of inference like line 2 above. He's supposed to be doubting everything, even his logic. So if this is what Descartes means, it doesn't do what he wants.
2. A performance: Hintikka suggests that the relation of cogito to sum is that of process to product, not premise to conclusion. If this one appeals to you, great. Personally, I've got absolutely no clue what he's on about.
3. A syllogism. The argument would run something like this:
1 There exists an object called 'I' with the property of 'thinking now' ( I am thinking)
2 There exists an object 'I'(I am)
This seems to be a valid argument. But wait, there's some cheating going on. The first claim contains everything you need for the conclusion. If the thinker knows he exists, why go through the whole rigmarole of mentioning thought in the first place? Why not just say "I am"?
In fact, doesn't the whole situation make the assumption that "I am" in the first place to get to "I am thinking"?
Thomas Hobbes makes a similar objection to Descartes. He asks, why not "ambulo ergo sum", "I am walking therefore I am"? Why single out thinking?
Descartes responds to Hobbes, granting that "I am walking therefore I am" is perfectly valid, but not useful. The point of the cogito is to establish an indubitable position. Descartes says it's possible to doubt you are walking, but how could a person doubt he was thinking?
It's important that by thinking, Descartes didn't simply mean sitting around pondering, but any mental activity like feeling and believing. So by doubting that you are thinking, you are actually thinking. This is Descartes' point. You can't genuinely doubt that you are thinking, so you can't genuinely doubt your own existence.
Descartes is being a little cheeky here, helping himself to some logic that his Doubt shouldn't let him have. That in order to doubt something, there needs to be a "thought" by his definition is fair enough, but it may not necessarily be the case that thoughts have to have a thinker. So the argument would lose its power and become
1. It seems that I am thinking
2. There are thoughts
But it seems a bit of a quibble; most people are prepared to accept that thought have thinkers as an analytical truth, and let Descartes' central argument stand.
The Meditations are all downhill after that. Descartes uses the cogito as his foundation for a whole bunch of proofs of the existence of God and the outside world, and he strays fairly far from his systematic Doubt and his basic claims to do it.
Descartes gives us the precursor of Snell's Law of refraction and invents the X-Y co-ordinate system we commonly use. But the cogito stands as his most renowned contribution to the world, famously misquoted and misunderstood for generations.
originally learnt Andrew Pyle's and David Bain's notes and Bernard Williams's Descartes: The project of pure enquiry 1978, among other sources.