 Typically, epistemologists are concerned with the conditions under which a subject may know or be certain that p at a particular moment. Interestingly, however, somewhat different issues arise for certainty over time. As this was a primary concern for Descartes, who tells us in the First Meditation that he wants to establish something “in the sciences that was stable and likely to last,” we can best see how those issues arise in the context of Descartes’s epistemology (PW 2, p. 12).