 Descartes allows that if we are careless or not paying attention, we can entertain the supposition that “God” does not exist or is a deceiver, but he holds that upon reflection the supreme being is a necessary existent, and a necessary existent that cannot deceive (The Fifth Meditation, CSM 2:46–48). We might have thought otherwise at the start of inquiry, before we had carefully unpacked and examined our concepts – at a moment when we were not yet putting forward a priori non-sensory claims, which are the only sort of claim that are suitable as premises in a metaphysical argument. As often happens in philosophy, we come to recognize that there are things that seem clear to us before we have thought them through, and we recognize that the reason why they seemed clear was that we hadn’t thought them through yet. As Descartes himself writes, 