The problem of skepticism is the fundamental epistemological problem Descartes addresses. He introduces three forms of it, each embedded in a possible error-scenario. The first possibility is that, since my sense perception is sometimes misperception, my sensory experience in any given case may not reflect how things are outside my experience. The second possibility is that maybe I am dreaming when I think I am awake. And the third possibility is that maybe I am deceived in all my ideas and beliefs by a powerful demon. The third is the most radical, far-reaching, and potent of the error-scenarios Descartes discusses. Unlike the first two, it threatens knowledge of all kinds. So, if Descartes is to defeat skepticism in a fundamental and comprehensive way, he must eliminate that possible scenario.