Sigmund Freud (1900/2003, 1915/1968) proposes a mental apparatus based on two complementary ways of functioning: the primary process mode and the secondary process mode. The former refers to an associative mode of thought, typical of unconscious mental life. The secondary process refers to an elaborate and rational mode of thought, which uses elements of reality to select the items from the primary process that are consonant with it and inhibit the ones that are not (Freud, 1895/1956; 1900/2003). In 2000, Linda Brakel, Howard Shevrin and colleagues at the University of Michigan designed the “Geometrical Categorization Task” (GeoCat). They propose that the logic of Freud's primary process establishes similarity on the basis of superficial attributes (e.g. forms, outlines, colors, etc.), whereas the secondary process establishes similarity on the basis of a contextualized analysis of stimuli based on the reality principle, i.e. on the total configuration of the objects or words (Brakel, Kleinsorge, Snodgrass et Shevrin, 2000; Bazan, 2007a). The GeoCat is based on a cognitive theory of categorization (Medin, Goldstone et Gentner, 1990) proposing two types of similarity judgments: an attributional judgment (ATT) and a relational judgment (REL). The idea now is that a choice based on ATT similarity reflects a mental functioning on a primary process mode and that a REL (configurational) similarity reflects a mental functioning on a secondary process mode. Many empirical studies have been conducted with the GeoCat. They all have the same reasoning: each times the Freudian psychoanalytic theory predicts a predominance of the primary process mode, more ATT choices are expected on the GeoCat. Specifically, Freud (1900/2003) suggests that the primary process mode predominates when there is a diminished Ego function: this is the case in the Unconscious (Freud, 1895/1956); in early life when the Ego is not yet (fully) elaborated (Freud, 1900/2003); and in anxious regression (Freud, 1895/1956; 1926/2015) or in psychosis (Freud, 1895/1956; 1900/2003) where the Ego-functions are structurally instable. So far, the results with the GeoCat show a significant dominance of ATT responses in all the cases proposed by Freud (respectively Brakel et al., 2000; 2002; Brakel & Shevrin, 2005; Bazan et al., 2013). We will develop more precisely the case of primary process mentation in psychosis through a couple of empirical studies (Parrent, 2011; Bazan et al., 2013).