Thought may be defined in general as mental activity, conscious or unconscious, based on the various modes of representation, including the most archaic. More narrowly, the meaning of thought may be confined to ideational activity, dependent on the faculty of judgment and on the faculty that brings into conjunction images of things and images of words. The discussion here will be restricted to the narrower conception of thought as ideational activity, but always bear in mind that the narrower meaning is deeply rooted in the more general one.

Freud approached ideational thought from three different angles, which did not necessarily overlap. The first was the "psychological" approach, as outlined in the "Project for a Scientific Psychology" (1950c [1895]) and further developed in The Interpretation of Dreams (1900a), "Formulations on the Two Principles of Mental Functioning" (1911b), and "Negation" (1925h). In this...