To define existence? No.
To illustrate the petitio principii nature of ontology, and indeed logic and philosophy in general (ie they are “based on themselves”)? Certainly.
Incidentally, on the subject of rocks not exist: A->B does not imply ~A->~B. (This logical fallacy is called Denial of the Antecedent.)
Help, I’m turning into Libertarian!!!
It may soon be difficult to determine whether a computer is consious or not - already programs have been written that can fool many people into thinking that the computer that runs that program is human and/or conscious-
give it ten years, and a ccomputer that says I think therefore I am may have such a comprehensive repertoire of responses that no-one (but the programmers) would be able to catch it out-
yet it would be no different when it was switched on to when it was switched off.
Would such an entity be conscious? Would it necessarily therefore exist?
It is possible that we are no more than programs which mimic consciousness ourselves. IMO.
We’re not talking about anything tangbile here though, are we; ‘I exist’ is not the same as 'This arrangement of matter exists and is me"?
Exactly. I look at Descartes’ statement as self-evident. Whatever thinks, exists. If it’s an illusion, then the illusion exists.
Yes, so it boils down to ‘X does Y’ in which X must exist in order to do Y.
it does seem circular to me. propositionally:
(there exists an i which thinks) -> (i exist)
there exists an i which thinks
:. i exist.
to say be able to meaningfully say “i think” we must already assume the existence of i.
it seems to me pretty presumptuous, actually, to assume there is such an i. that there seems to be can’t be denied, but to assume so is to assume dualism.
I think, therefore I think I am.
I don’t think it’s presumptuous at all; in order for X to do Y, X must exist.
I think I think,
therefore I think I am
I think [I think I think], therefore I think [I think I am]
I think [I think[I think I think]], therefore I think [I think[I think I am]]
And so on…
Although to be quite honest, I’m not terribly convinced that I do actually think.
Certainly the word think doesn’t mean anything to me now, after typing and subvocalising it so many times.
Axioms do not have the word “therefore” in them.
I think the real argument being debated here suffers from being verbalized. The sentence, “I think, therefore I am” may presuppose the existence of “I”, but I believe that’s just an artifact of translating the argument into a coherent sentence. The real meat of the argument can get loss in analying picky details of the sentence structure. I, and I assume the rest of you, are constantly experiencing various sensations. We don’t need to presuppose anything to know this is true, the experiences themselves are all that’s necessary. And to have these experience, there must be something that is “me”.
Think of the argument without the words getting in the way.
Like Cabbage said, you’re getting too hung up on the phrasing. Even though it has the word “ergo” in it it is still not really a proof but an axiom. A verb requires a subject. “Thinking” requires a thinker. This is self-evident. It does not require proof.
Well, IIRC, you’re a math guy. How about the definition of numbers? Do they assume the existence of numbers?
Noting a begged question is not “nitpicking”. 
I don’t know if you need to presuppose anything, but you did. 
There must be a subject of experience. Very well, we now know how to use the verb “To experience”.
Think of a newborn baby. Would we say its existence is intuitively obvious to itself? Would a cow say “cogito ergo sum” if only it could?—for of course cows have experiences. (don’t they?)
Hang on though; I think therefore I am does not necessarily need to work in reverse and from outside ([it is, therefore it must be thinking], or even [it appears to be thinking, therefore it must be aware that itself exists])
It’s a solopsistic axiom. It only applies to “I.”
The concept is that, though you can reasonably doubt almost everything, by the time you’ve finished all that, there’s no way to doubt the fact that you’re doubting a lot of stuff. By attempting to observe your own thought processes, you establish a reality in which you are an observer, and, therefore, ‘are’.
As for wether a rock or cow thinks; there’s no way to tell from the outside; the observer can only validate its own existence to itself. Even things that more actively claim to think (like, say, people) might be figments of my own imagination; it is only when I try to claim that my imagination is a figment of my (non-existent)imagination do I fail.
Of course it needn’t work that way, Mangetout, the reason I suggested it was because of Cabbage’s exposition about others.
Diogenes, it doesn’t matter to me if it is solipsitic or not, it still assumes what it attempts to prove. I don’t mind the axiomatic assumption “I exist.” If that’s all “cogito ergo sum” was then it wouldn’t have made it into the popular consciousness.
begbert2: “As for wether a rock or cow thinks; there’s no way to tell from the outside” Then either we misuse the verb “to think” or the same can be said of everyone (the problem of other minds).