The Way We Divide Forms ’in Our Soul’: Conceived Parthood at Plato’s Sophist 250b8

What does Plato mean when he declares at Soph. 250b8 that Theaetetus is positing Being in his soul (ἐν τῇ ψυχῇ τιθείς) as a third something encompassing Change and Rest? Is he merely clarifying that the act of positing is a mental act? Or is he making a further point? This paper argues that the locution ‘in the soul’ plays a significant role in the passage in alerting to a contrast between the way Being and its relation to Change and Rest are intelligible to Theaetetus and the way Being really is and relates to those two other kinds. This interpretation is set against another interpretation, defended by Leigh, according to which the phrase should be understood as drawing a contrast between what is done by the agency of the soul and what is done by the agency of the body. The paper then explores the consequences of the proposed interpretation for our understanding of the broader context of the passage. In particular, it argues that the claim that Being is a third something encompassing Change and Rest is more problematic than critics have usually assumed. To account for Theaetetus’ depiction, it develops the notion of ‘conceived parthood’, which are part-whole relations posited by the mind for the needs of a philosophical enquiry.