In Aristotelian Scholasticism, we find the notion of a real quality, a quality that is a res, a thing in a technical sense that is more than a mode, or, more broadly, the notion of a real accident. Such a quality is really distinct from its subject and separable from the substance it inheres in by God's power. In Descartes’ words, it has its own act of existence (AT III 667, CSMK 219). A prominent reason why such qualities were accepted was the view that transubstantiation requires the separability of qualities. In the sacrament of the Eucharist, Christ's body and blood are substituted for the bread and wine, but the accidents that pertain to the bread and wine remain: Arnauld listed “extension, shape, color, smell, taste and other sensible qualities” (AT VII 217, CSM II 152–53). Arnauld anticipated that Descartes’ view that all qualities are modes would raise objections on account of the Eucharist; this issue contributed significantly to his works being placed on the Index in 1663. Descartes denies that the church requires the notion of a real quality; instead, he surmises that theologians who first tried to explain transubstantiation were already committed to belief in real qualities (AT VII 252–53, CSM II 175–76).

And indeed there were also philosophical motivations for regarding certain qualities as res. Thus, Ockham argued that the question whether a quality is a res depends on whether a change in a type of quality can be understood in terms of locomotion. If not, it is a res; thus, heat is a res, figure is not. The Scholastics disagreed about how many types of qualities count as res (Adams 1987, 277–85).

Interpreters often fail to distinguish real qualities from substantial forms, but in Scholasticism substantial forms underlie qualities, and they are incomplete substances, not accidents. Descartes also distinguishes them. His main objection is that the notion of a real quality is incoherent, because it presents entities as being qualities and substances at the same time. It does so because it presents the qualities “as having an existence distinct from that of body” and as separable (AT III 667, CSMK 219; AT VII 441, CSM II 297; AT III 67, CSMK 219; AT VII 434–35, CSM II 293; AT VII 253–54, CSM II 176).