In the example you yourself gave of 'I think therefore I am', the claim following 'therefore' is a conclusion, not a premise. So no, Descartes didn't start with the premise that he existed.
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.
Well, meditations wouldn't be a bad place to start. And philosophers differ a lot in their views of teh significance of philosophy. I regard it as self-indulgent, but interesting to those so inclined. Like Mathematics, it's an a priori subject anyone can do from an armchair. Unlike Mathematics, it isn't that useful for science. But unlike mathematics, it will help you achieve understanding about the structure of how you ordinarily think about the world (if not the world itself).
You can construe the cogito as an argument:
P1 I think.
P2 If I think, then I exist
C1 I exist.
P1 and P2 are premises, C1 is derived.
The interesting point is that P1 is not arbitrary, in the sense that if one grasps it, then it is true.
That's a really interesting property for a claim to have - especially for a contingent claim to have (e.g. 2+2=4 is necessarily true, and so if one grasps it, trivially it is true, but that's not interesting in the same way).
Further, D thinks that if one grasps it, then one knows one grasps it, (and that it is true).
So for D we have a priori certain knowledge of a contingent claim.
If course, you can question 2, and some people have. Quine is one person to read on that.
Re 'what does it mean to exist' - what do you mean by that? Your criticism demonstrates some ignorance of Descartes, as Descartes makes substantive claims about the nature of his existence as revealed by the reasonining in Mediations. For example D would say he knows that he exists as a thinking subject - a substantive claim about his nature. He goes on to make lots of controversial claims about his nature (some of which get us to Cartesian Dualism).
I wouldn't downvote anyone, but perhaps because it's a paradigm example of a particular strain of thought you see a lot amongst techie/engineering types with limited experience of philosophy, and a line of thought that doesn't stand more than a cursory examination.
You've already answered it with your neurons example. Someone else has pointed out that neuroethology will not touch on normative facts (although will presumably uncover empirical facts that have normative significance).
People have been predicting the death of philosophy for hundreds of years. It hasn't happened yet, and it is unlikely to, as there are always fundamental questions to ask (even though progress on them is difficult, if possible - but that's another question).
I think you underrate this line of thought. The existence of neurons is, of course, not justified in the philosophical sense - but its hard to argue that anything is justified in the philosophical sense. Reason might be a source of justification, but for that to be so you first have to assume that reason is justified. Senses might be a source of justification, but first you have to assume that knowledge derived from senses if justified. If you do not make either of these assumptions you have to retreat form justified to probabilistic or tentative knowledge, and in those terms the existence of neurons is a rather more reasonable assumption.
It's not like philosophy is interested in some special notion of 'justified' - philosphy tends to be interested in our ordinary notion of justification.
We normally think that the belief that neurons exist is justified. Philosophy will ask what that amounts to. It may turn out that the belief is unjustified - but most philosophers will reject this. They will argue over different accounts of what that justification consists in.
Modern philosophy has indeed moved beyond this notion of justification, but for a long time it was the central project of Western philosophy. What were the rationalists, empiricists, Kantians, and positivists arguing about other than the justification of knowledge (in this sense).
EDIT: Maybe I'm going overboard here in attributing to philosophy in general what is only true of epistemology, my favorite branch of it. In that case, I should have been referring to "epidemiological justification".
There has been a strand of 20 cent philosophy that has regarded justification as that which together with truth and belief yields knowledge - if you do that, you're individuating justification in terms of a particular epistemic role, and that so it's a theoretical notion. That might be what you're getting at.
But most philosophers nowadays use the notion of 'warrant' for that. So for them, the notion of justification doesn't have that particular theoretical role.
Generally, why be interested in a theoretical notion? The fundamental questions in Epistemology are questions like:
* What is knowledge?
* What is justification?
* Which beliefs are justified?
* What do we know, and how do we know it?
etc etc
Those are the starting points, and they are phrased in English. They use ordinary notions. Given that, no answer to them which first redefines the terms to yield distinct questions is going to suffice as an adequate answer.
Philosophers use a lot of technical jargon, and are interested in technical questions. But generally, those questions have pretty clear connections to our ordinary ways of thinking about the world.
