 The philosophical background to the dilemma is this. If you are a realist about scientific knowledge and truth, as Aristotle is, the structure of your scientific theories must mirror the structure of reality, so that scientific starting-points or first principles, must also be the basic building blocks of reality. Suppose that this is not so. Suppose that your physics tells you that atoms are the basic building blocks of reality and that your psychology tells you that sense-perceptions are the starting-points of scientific knowledge. Then you will face a very severe problem, that of skepticism. For a wedge can be driven between the starting-points of scientific knowledge and reality’s basic building blocks. René Descartes’ famous dreaming argument is one familiar form such a wedge might take. Your sense-perceptions are consistent with your being always asleep and having a very detailed dream.