Rene Descartes, in his Third Meditation, argues that the only possible explanation for his having an idea of God is that God actually exists; and that his idea of God is an innate idea placed in him by God. The details of Descartes's argument need not detain us now — except to comment that the scholastic elements in it put it more or less beyond the pale of contemporary analytic philosophy. Nevertheless, something strikingly similar, in broad outline, can be found in a book that is very much of our time and squarely within the analytic tradition, namely, Thomas Nagel's The Last Word (1997). Nagel reflects on our use of reason — ‘a local activity of finite creatures’ — to arrive at the idea of infinity. Ludwig Wittgenstein would recoil from a realist model of the relation between our grammar of the infinite and the infinite itself. Both Aristotle and Wittgenstein recognise a fundamental and intimate relation between time and possibility.