Perhaps nowhere in this book is the Sulmo principle more steadfastly used than here. The principle states that “a philosopher's system must not be reconstructed and judged until after his death.” In order to reconstruct his system, we may bring together elements that the philosopher himself did not bring together – for lack of time, or insight, or whatever. Such a reconstruction is permissible so long as it is consistent with his major views. Only such a picture is worthy of being examined.

I have brought together theses that Descartes maintained at various periods, and none of which – at the very least, no central thesis – he recanted. For example, I have brought to bear on the cogito Descartes' theory of deduction. The theory was cast in Rules for the Direction of the Mind, written as early as 1628, perhaps even earlier, never discussed in detail again, and never published during Descartes' life. Even before it was finally published in 1684 in a Dutch translation, it had already profoundly influenced Logic or The Art of Thinking, famously known as Port-Royal Logic, first published anonymously in 1662. The problem I have set myself is to determine the truth-value of a counterfactual. It goes something like this: If Descartes had given us in the first meditation (or in the first three parts of Discourse on the Method) his theory of deduction, his metaphysical doubts, the doctrine of the creation of eternal truths, and the theory about memory, could Descartes have regarded the cogito as an argument?