And this subject is quantity (an actually existing quality is always spatially extended, not in virtue of itself, but accidentally so in virtue of quantity). Accordingly, in a fully-fledged Totality (intrinsic being) the quality is, albeit mediately so, carried by substance.
See for this St Thomas in Summa Theologiae, I.II. q.53.2.ad 3 : Nam accidens significatum in abstracto importat habitudinem ad subjectum, quae incipit ab accidente et terminatur ad subjectum. Nam albedo dicitur qua [ = the auxiliary being] aliquid [ = subject] est album.
Translation : For an accident expressed abstractly entails a relation to the subject, which begins at the accident and ends at the subject. For whiteness is called that something in virtue of which something is white.
The difference from having the auxiliary being (accident) concretely expressed (thus as white instead of as whiteness) is that this relation is precisely the other way around, that is, beginning at the subject (carrier).
In the Summa (at the same place) thus : Sed in concretis incipit habitudo a subjecto, et terminatur ad accidens. Dicitur enim album quod habet albedinem.
Translation : But in the concrete [that is, in things expressed concretely] the relation begins at the subject, and ends at the accident. For one says the white [thing] which has whiteness.
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