The cogito is true. So what is a Thomist to do with it?
Notice that to say that the cogito is true is not to say that everything that Descartes does with it is true, or even that Descartes understands his own axiom in the most fruitful way. But the statement itself is indubitable, and with a long history of precedents. Its indisputable character is such that it clearly deserves to be taken as some sort of first axiom. But how are we to move forward from it?
Descartes valued the certitude of the axiom the most. Here St. Thomas would want to arrange the diverse senses of certitude or certain or sure, and he would insist on certainty or sureness belonging to things first, before the intellect. To be sure or certain is to be determined (more or less) to one outcome or result, and we can know this first of things before our own thoughts. The degree to which we can judge correctly about such determination is part of the question that gives rise to the cogito, but it is still in things that we first find the greater or lesser determination of outcomes.
St. Thomas would be more likely to take the cogito as manifesting something about existence or being, namely, that being is known first – and in an indubitable way – by our own reflection on our vital actions, and in the higher such actions before the lower ones. The most illuminating thing about the cogito when taken in this way is that is shows that existence is innermost in things. In seeing that one thinks, and therefore that he exists, existence is seen as the foundation of all action and activity. Thought becomes a certain manifestation or expression of existence, but existence is of itself, foundational, autonomous. It is even more interior than our own thoughts, even our own thoughts about our thoughts.
The cogito shows us existence is a principle, but not just any principle. It shows us existence is primarily the self or the individual (or in human nature, the person) and not just any foundation. Matter, for example, is a foundation and a principle, but it is not a principle constituting a self or an individual. The truth of the cogito, in fact, is incompatible with materialism, for it shows us that existence is primarily a self while it is evident that matter is not (nothing common to more than one self could be a self).