When someone says 'I am thinking, therefore
I am, or I exist', he does not deduce existence from thought by means
of a syllogism, but recognizes it as something self-evident by a simple
intuition of the mind. This is clear from the fact that if he were deducing
it by means of a syllogism, he would have to have had previous
knowledge of the major premiss 'Everything which thinks is, or exists';
yet in fact he learns it from experiencing in his own case that it is
impossible that he should think without existing. It is in the nature of our
mind to construct general propositions on the basis of our knowledge
of particular ones. (2nd Replies)