Descartes Second Meditation needs no introduction. But here’s one anyway. After realizing that he could not vouch for the source of all of his various beliefs, Descartes sets out to clean house. He hatches upon a method: examine a belief, and see if one can raise any doubt whatsoever towards it, no matter how far-fetched. If the belief was doubtable, it admitted some possibility of being false, no matter how slim. Accordingly treat it as false and move on. In this way, if one maintained this harsh standard, one should stumble upon some proposition that is absolutely beyond all doubt; something certain that could be used as a first principle from which to deduce all knowledge.
Overwhelmed with the magnitude of the task of going belief by belief, Descartes hatches onto a clever thought experiment: suppose there was some infinitely powerful being whose only role in life was to use all of the immense power at their disposal to deceive you. With this assumption, giant clusters of beliefs can be hacked off in bulk.
But, now, in this second meditation, Descartes is beginning to worry that no belief can pass this rigorous standard of complete undoubtability, armed as it is with this deceiving genie test. However, just when all hope seems lost, Descartes realizes that no matter how powerful this being is, they cannot deceive him into believing he exists if he in fact didn’t (because then there’d be nothing to deceive). I am, I exist. This is the absolutely certain proposition. What follows is Descartes trying to determine what is the nature of this absolutely certain ‘I’. Crucially, he argues that insofar as it is certain, it cannot be grounded in anything uncertain: for example, the body. With this argument in hand Descartes can now leverage it to arrive at the immateriality of the mind/soul, and the two incommensurate substances for which he so famous.
It is impossible to overstate the influence (for the better, for the worse) of Descartes’ discoveries here in the Second Meditation.