He seems to assume that because he (what?) can think, he exists.
Either he has an implicit assumption about what it means to exist (which he never describes, and which is therefore useless to me), or his definition of exist is "can think", in which case the famous proof would be trivial and useless (it would boil down to "I exist therefore I exist").
I already see many people here don't understand what I mean, but this is the kind of thing that makes most philosophers useless to me.
Such arrogance from someone who clearly has no understanding of what he critiques. I hate to add to the dogpile, but seriously take a step back and realize that maybe there's something you're missing rather than everyone else being fooled.
Your issue here seems to be "this doesn't answer the question I want therefore this is useless". This is just a lack of imagination on your part. Sure, he doesn't delve into the nature of what it means to exist, but "I think therefore I am" crosses a huge chasm in its own right. It establishes existence. One needs to do this before you can begin to discuss the nature of existence.
The "uselessness" of philosophy for most people, and I think this is where it fails for you, is that it provides far more questions than it provides answers. Philosophy doesn't package the world into a neat little box; it provides you with a framework to develop your own understanding of it. You still have to do much of the mental legwork yourself. Contrast this with science, where it does provide you with neat little bite-sized answers to satisfy ones superficial curiosity (E=mc^2? Cool!)
You also seem to think that Descartes needs to provide an accont of what it means to exist. Why think that? You speak English. 'Exists' is a perfectly intelligible English word. And so we can understand the conclusion and the explicit assumption.
Of course, it is an interesting philosophical question as to what existence amounts to. Quite a hard one. But it's a different philosophical question.
You seem to be criticising Descartes for not answering that question, but that's just to conflate distinct issues.
It's also a little incoherent to say that philosophy is useless because it ignores certain questions, which then turn out on inspection to be further philosophical questions.
And philosophers do grapple with what existence amounts to. Go read some Quine if you're actually interested.
In fact, Quine provides an account of existence which vindicates teh implicit premise I've outlined above. But Quine is writing 400 years after Descartes, after the great philosophical revolution brought about by the logical developmenets of the philosopher Frege (which incidentially had an impact on computing and so on). So we might say that Descartes doesn't answer the questions you raise fully. But only someone a little up their own arse would criticise Descartes for failing to deal with questions it took 400 years of philosophical, logical and mathematical development to make substantive progress on.
Either he has an implicit assumption about what it means to exist (which he never describes, and which is therefore useless to me), or his definition of exist is "can think", in which case the famous proof would be trivial and useless (it would boil down to "I exist therefore I exist").
I already see many people here don't understand what I mean, but this is the kind of thing that makes most philosophers useless to me.