Why did Descartes pick thinking of all possible attributes to logically establish existence? Rocks exist but don't think. What exactly did he have in mind to establish? Was it really existence? Did he have any valid reason to doubt his or our existence? Wouldn't pain be a better criterion? Or movement? Or change? If a non-philosopher raised such a question we would certainly look askance at him and not value his "evidence" either way.

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Thank you for this inquiry. You are on to a very important point. First, some thoughts on Descartes:

Descartes set up the ultimate skeptical project: In an age of the emergence of modern science, he asked what we can really have unmistakable certainty about? To take your example, can we have absolute, uncorrectable (incorrigible) certainty that the rocks we see and study are as they appear? He proposed the massive skeptical hypothesis: Can we rule out that there is an all powerful evil genius who is making us appear (again, using your example) to see, observe, and study the movement, change, and location of rocks when, in fact there are no such rocks? In contemporary popular cultural terms, can we rule out that we are in the Matrix? Or to use terms that were popular in the 1980s, can you rule out that your brain is now in a vat at MIT and electrochemically stimulated such that you are having all the experiences you have now and so you are in a kind of virtual world, but not an actual world? Descartes argued that even under such dire conditions you or I could still be certain of one thing: I exist. That is because one's own existence is known with such certainty by oneself, that the existence of the self is presupposed in either the doubting or believing that the self exists. If I doubt I exist, I must exist otherwise I could not doubt this.

Over to your suggestions: A follower of Descartes might be very happy with your example of pain. It seems (to many of us) to be impossible for there to be pain without there being some thing that is in pain. And arguably, when a subject like you or I are in pain, we know not just that there is pain, but we have an awareness that you or I as subjects are in pain. Some philosophers (including David Hume) may have doubted this, but that seems quite contrary to common and natural experience. So, I think you can appreciate that Descartes and his followers would enthusiastically welcome your and other reasons for recognizing the sturdy certainty that we may have that we exist as subjects, but his target is the absolute or near skeptic who is going to doubt as much as she or he can. When you point out to such a radical skeptic that you see and measure the movement of rocks, she will ask how you can be so certain you are not doing so in a kind of vivid dream, virtual world.

The first thing to observe is that Descartes felt that the notion of 'unconscious thought' was incoherent. Maybe there can be neurological processes going on in the brain that we're not conscious of -- he would have no quarrel with that -- but, simply in virtue of the fact that there's no consciousness involved in the case, he would deny to these the title of 'thoughts'. If something is going to qualify as a thought at all, it needs to be a conscious thought. But then, what does 'conscious' mean? It means that these thoughts, when I have them, are accompanied by knowledge. And knowledge of what? Of the fact that I'm having them.

"But", writes Descartes in the Second Meditation, "I do not yet have a sufficient understanding of what this 'I' is, that now necessarily exists." It's very easy to show that something exists: for, as soon as we think, we have this conscious awareness of the fact. But what exists? Answer: a thinking thing! And so that is what Descartes takes the pronoun 'I' to refer to. Now, it may very well be that there's more to my nature than just the fact that I'm thinking. Maybe I also have physical attributes; maybe not. Descartes will certainly be exploring that question later on, but he's not yet in a position to tackle it. But he can at least say this much: regardless of any other attributes that I may or may not have, I can be consciously aware of the fact that I'm thinking, whenever I think. If, for a time, I was to stop thinking, I would no longer be aware of anything, whether my own existence or anything else. Indeed, I would, in that regard, be like the rocks you mentioned. It's true that rocks do exist but don't think: but they can't have any conscious awareness of the fact that they exist, precisely because they don't think. There's the difference. Descartes not only exists: he also knows that he exists. He's conscious.

And so (you ask) what exactly did he have in mind to establish? It was actually a little bit more than just his existence, for it also involved his nature. What he sought to establish was his own existence as a thinking thing. And then, further explicating what he meant, he spelled it out: "But what then am I? A thing that thinks. What is that? A thing that doubts, understands, affirms, denies, is willing, is unwilling, and also imagines and has sensory perceptions." Descartes used the word 'thought' as a blanket term to cover many different kinds of psychological activity, but the one thing that (as far as he was concerned) linked them all was this conscious awareness that necessarily accompanied each one.

How about using pain as a criterion? Sure, that'll work. Given that pain is conscious -- I can't be in pain without thereby knowing that I am in pain -- it can certainly be used to establish that something is in pain. But that's not a different argument: it's just the same argument over again. Given that pain is conscious, and bearing in mind the broad way that Descartes used the term 'thought', pain will qualify as a thought just like any other. But what about movement or change? Well, insofar as we can have a perceptual awareness of movement or change, we can use that too -- because that too will just be more thought. But what about unperceived movement or change? Well, in that case, there may very well be something existing there. But the difference is that there won't be any awareness of that existence. The moving object itself must certainly exist in order to be moving: but it won't be aware of the fact that it either exists or moves. If it was to become aware of that, well, then we'd be dealing with yet more thought, and we'd be back with the original argument again. Descartes' argument isn't just about the truth of a conclusion: it's about discovering that truth. And to discover anything requires thought. That is to say, to be aware of something requires... being aware of something. And it's that last point that best highlights the sheer immediacy of Descartes' argument. Other arguments, to other conclusions, might involve a great deal of rigorous and abstract and difficult thought: but this one just rests on the mere existence of any thought at all. As soon as we think, we're aware of the fact: because thought, in essence, is awareness.

Descartes' famous line, "I think, therefore I am", is celebrated more for what it symbolises about his wider philosophical methodology, than because it establishes any especially momentous conclusion, or because he argues it in an especially elegant way. In fact, it's not really an argument at all, in the sense of the derivation of a conclusion from a premise. (In the Second Replies, he points out that it's more to be thought of as "a simple intuition of the mind"). All that it is, fundamentally, is awareness. And, for a philosopher, that seems like a pretty good starting point!

Aha! My answer seems to have crossed in the mail, as it were, with Charles Taliaferro's. Well, there you go, two for the price of one! The price, of course, being free: isn't this a lovely site?

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