Just think of it. Many hypotheses can explain even a single sensory experience. Suppose that I'm having this auditory experience, I probably think there is someone knocking three times at the door. But there might also be three people knocking at the door one after another, or someone who likes practical jokes, might play a sound recording of door knocking behind the door, or perhaps a dangerously brilliant neuroscientist is stimulating my brain in precisely such way that I experienced a room, a door and door knocking whereas none of these really exists. To distinguish them from deductive and inductive inferences, Paris has introduced a notion of abduction to refer to such explanatory inferences. Inferences from sets of experiences to explanations of these experiences. Abductive inferences are very important. In fact, they allow us to solve problems. However, as my example illustrates, they have the disadvantage of being uncertain. There are many hypothesis that can explain even a single sensory experience. Scientists do not waste much time on far-fetched alternative explanations. They only consider serious candidate explanations, perform real experiments to eliminate all but one of them so that's at the end of their investigations, they can make an inference to the best explanation. But still an abductive inference, so it's still uncertain, but it's the best one we can reasonably hope for, and for scientists, that's enough. It's not enough for philosophers though. On the contrary, philosophers are very keen on performing what they've called towards the end of week 1 abductive thought experiments. Like in my example, they love to use their imagination to think of various alternative explanations. Some of these alternatives explanations are completely innocent, they are not dangerous to your mental health. For instance, that there may be three people knocking at the door is something that I can easily check or verify. All I have to do is open the door. [inaudible], what are you doing here? You're supposed to be. However, some of these alternative explanations constitute skeptical alternatives. Alternative explanations that cannot be ruled out. An example is the dangerously brilliant neuroscientist who is stimulating my brain in such way that I have exactly the experiences that I have. If I checked whether there is one or three people behind the door, whatever results I would experience. These experiences could be perfectly well explained in terms of the actions of the dangerously brilliant neuroscientist. Skeptical alternatives may be used to define the problems of skepticism. One of the first philosophers to do so was Descartes. Hence, this way of defining skepticism is called Cartesian Skepticism. In his first meditation, Descartes is exacerbating doubt to its ultimate ramifications by means of two abductive thought experiments. The first one is his famous dream argument. It's conceivable and thereby possible, he argues, that all my sensory experiences of an alleged external world are part of a long consistent dream. It would be weird, it might be very improbable, but there is no way whatsoever that that could exclude such possibility. Therefore, I cannot be sure on the basis of my sensory experiences alone, that there really exists an external world. The skeptical possibility of a long consistent dream undermines everything I think I know including the existence of the worlds. Although the dream argument calls the existence of the external world into question, it doesn't affect a priori knowledge. Like for instance, mathematical knowledge, knowing that 3 plus 2 equals 5. Since Descartes wants to define skepticism on a razor's edge he introduces a second thought experiment, his evil demon argument. It's conceivable and thereby possible. He argues that there exists an evil demon who is systematically misleading me in literally everything I believe. Not only about the existence of the external world, but also in mathematics. The evil demon makes me strongly believe, feel absolutely certain that 3 plus 2 equals 5, whereas in fact, it doesn't. Again, I cannot tell, I cannot exclude that possibility. Now everything I think I know, including my mathematical certainties are undermined. Now what are the problems of skepticism that the Cacts and Cartesian skeptics defined by means of skeptical alternatives. Firstly, abductive thought experiments in which the possibility of skeptical alternatives is entertained, calling into question the existence of the external world. That is called the metaphysical problem of skepticism and we'll consider some solutions to it in the next clip this week. Secondly, skeptical alternatives also define the epistemological problem of skepticism, calling into question the very possibility of knowledge. S cannot know that P, because S cannot rule out any of P's skeptical alternatives. The third and final clip of this week, we'll consider some solutions to this epistemological problem. Skeptical alternatives also define further problems of skepticism. The possibility that we may be continuously talking past each other, because being based on our private experiences our concepts may have nothing in common constitutes the conceptual problem of skepticism. The possibility that you may be a robot or a zombie because in contrast to my direct access to my own experiences and consciousness, I don't have access to your experiences and consciousness gives rise to the problem of other minds. Finally, solipsism is the very lonely view that my mind is all that exists in the world and that it confabulates everything and everyone else.