The attempt to draw a clear distinction between Philosophy and the empirical sciences can almost be taken as the defining trait of the analytic movement in contemporary philosophical thought. The empirical science that has most frequently threatened to swallow up questions of particular interest to philosophers since the time of Descartes has been psychology. Characteristic, then, of analytic philosophy has been the rejection of what it terms psychologism, that is to say, the mistake of identifying philosophical categories with those of psychology, whether introspective or behavioristic. It is clear that to launch an attack on psychologism, thus conceived, presupposes that one has a list of philosophical categories which one is able to identify as such; and this in turn presupposes an ability to sketch, at least in a general way, a distinctly philosophical account of these concepts, although a systematic account along non-psychologistic lines may be a distant and ill-defined goal. The analytic movement in philosophy has gradually moved towards the conclusion that the defining characteristic of philosophical concepts is that they are formal concepts relating to the formation and transformation rules of symbol structures called languages. Philosophy, in other words, tends to be conceived of as the formal theory of languages. From this standpoint, consequently, psychologism is conceived of as the psychological treatment of concepts which are properly understood as formal devices defining a mode of linguistic structure.