⁵⁴ Descartes’s definition of substance is given in Principles, I. 51, and those of mode, quality, and attribute in I. 56. Descartes’s equation (p. 29) of ‘modes’ and ‘accidents’ elides a scholastic distinction between these two concepts: an accident has an aptitude or propensity for existing in a subject, a mode is actually inhering in one. On the differences between Descartes’s concept of substance and that of the scholastics, see Jorge Secada, ‘The Doctrine of Substance’, in BGDM 67–85 (esp. pp. 74–6). On the notion of ‘real’ in Descartes, see Stephen Menn, ‘The Greatest Stumbling Block: Descartes’s Denial of Real Qualities’, in Ariew and Grene (eds.), Descartes and his Contemporaries, 182–207.