 One of the most tantalizing questions concerns the relationship between mental representations (phantasiai) and lekta. The Stoics effectively endorse a version of Brentano’s Thesis that intentionality is the criterion of the mental, which demarcates the domain of psychology. The Stoics think that representations are what set animals and humans apart from plants and inanimate substances: only things with psuchē or soul have mental representations (phantasiai) of the world and make efforts (hormai) to move and change things (Origen On Principles 3.1.2, 196.12–197.8 Koetschau). But they also think that representations are a pervasive feature of psychological phenomena: every mental state either is a representation or involves one essentially, because it is an assent to a representation as correct or rejection of it as incorrect or withholding assent from it altogether. So representation (phantasia) is a feature of all and only mental states. Lekta constitute the content of these representations. The Stoics in fact define them as “what subsists in correspondence with a rational representation,” where a rational representation is one that belongs to rational animals like ourselves (Diogenes Laertius VII 63; Sextus Empiricus Adv. math. VII 51).