 As Mill declares as the beginning of his System of Logic, logic has two parts: the ‘science of reasoning, as well as an art, founded on that science’ (1843, 4). The art of reasoning is prescriptive; it provides us with the rules according to which we ought to reason. The science of reasoning is a descriptive-explanatory psychological discipline and analyses mental processes. The overall goal of logic is ‘the guidance of one’s own thoughts’ (1843, 6). The crucial question here is of course how the dependence of the art of reasoning upon the psychological science is to be understood. The mere fact that the art is prescriptive does not yet make it independent of psychology.