I have to say I think about nothing all the time, both in the sense of not thinking about anything, and in the sense of contemplating the concept nothing . P.L. Heath has a very fine piece on "Nothing" in the Encyclopedia of Philosophy . On the other hand I suspect that śūnyatā is not really emptiness or literally nothing - śūnyatā is a kind of non-substantiality, certainly, but in Western metaphysics that does not mean a non-entity. The point is that śūnyatā is emptiness, but of the detritus of external influence, or void or outside dependence. It has its own quality. It is also like a kind of expectant fulness, empty as the rich expectation of a joyous future event is in an obvious sense empty (of the event) compared with the experience of the event itself. A quality however is an entity, though not a substance.

There are plenty of philosophers who have not agreed with Descartes' line of thought here, though they are not "ancient" philosophers, as Descartes did not propound the "proof", if that is what it is, until 1637, in the Discourse on the Method and, in a slightly different form, in 1641 in the Meditations . You can find some interesting material in the "Objections" to the Meditations , for example the Fifth, by Gassendi, or the Fourth, by Arnauld. Hobbes too, in the Second Objection, makes of Descartes' argument a triviality. How (he asks) can I know that I am thinking? 'It can only be from our ability to conceive an act without its subject. We cannot conceive of jumping without a jumper . . .'. Through the centuries Descartes' dictum has come under even more fire from different directions, for example from A.J. Ayer in Chapter 2 of Language, Truth and Logic . Descartes was only entitled to say that 'There is a thought now,' not 'I think', i.e. 'There is an I and it thinks', because...

Some random suggestions: (1) David Pears pointed out that even if Kant's argument were wholly clear and wholly successful, which it is not, it could only show that existence is not an ordinary predicate, if it is a predicate. His view is that it is a predicate, just a very peculiar one; (2) There is also the view of the celebrated logician, mathematician and philosopher Bolzano, who writes in the Theory of Science ("Kinds of Propositions") that 'I take being [Sein] or actuality [ Wirklichkeit ] to be precisely what language makes it out to be, namely an attribute; whoever denies this confuses (I believe) actuality with substance. By substance I mean an actuality which is not an attribute of another actuality; hence I admit that we cannot truly predicate the putative abstractum of the substance (substantiality) of any object. For it is part of the concept of substance that there is no property of this kind. But it is not the same with actuality, which I consider to be a mere attribute, not...

Why should the redness of a red object not be a fact? We say of this tomato here, "Look, it's red." We know this proposition is true because we can see that the tomato is red, just as we know that the tomato is heavy - heavy for a tomato, anyway - because we can weigh it in our hand. The same thing applies to shape, supposing that we come to know the shape of something by visual inspection rather than by measurement. Now if our red object is viewed in green light, it turns black, because the light with colours at the middle of the spectrum, the green light, is complementary to the red light that the tomato "reflects", if we can say this. (In what way is a tomato not like a mirror?) The red tomato "absorbs" the green light. I think that your question goes deeper, however. This redness of the tomato might be thought not to be a physical fact, if you believe those philosophers who are impressed by the existence of an "explanatory gap", as it has come to be called, between physical and phenomenal...