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...

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?

I'm with Thomas Pogge on what the real issue is here. For what it's worth, I'm also no friend to the Ontological Argument. But let's see if a supporter of the argument might have something to say in response to this challenge... First of all, what form of the argument are we going to consider? It's been presented in many different ways over the centuries, and some versions have had much more force to them than others. In its simplest form, the argument basically goes as follows. God, by definition, possesses every perfection; existence is a perfection; therefore, God possesses it, i.e. God exists. But this version is notoriously vulnerable to objections like those that, for instance, Kant formulated way back in the eighteenth century. But there are other versions. One of the best (particularly associated with Leibniz, but formulated by several other people too, both before him and since) basically goes like this. (i) God, by definition, possesses every perfection; (ii) necessary existence...

We need to distinguish between a posteriori and a priori arguments. Empirical sciences such as physics, chemistry and biology are predominantly supported by a posteriori evidence, grounded in experience. By contrast, the 'exact sciences' like mathematics are predominantly supported by a priori demonstration, grounded in the pure intuitions of the intellect alone. Mathematics cannot be based on experience, it might be said, because experience can only ever reveal contingent truths, whereas mathematical speculation gives us an insight into necessity. But both a priori and a posteriori knowledge can jointly be distinguished from faith, which for present purposes we might as well just define as any other basis for belief that cannot be fitted into either of these categories (e.g. the authority of a respected individual or of a text regarded as sacred, or a private inspiration supposedly delivered only to the elect). Now, there have been many philosophers and theologians over the...