According to me, this exchangeability we need if we want to hold that accidents can be defined (because only then we can meaningfully speak of accidents), and this definability is possible not until there is finitude in both directions, that is, also in the downward direction, meaning that that something of which we first predicate the accident must be an individual, that is, a(n) (individual) substance. Therefore the sequence starts with (say) ' Socrates is white ' (and this term [white] is always concrete), moreover (it so starts), because substance must enter the definition of the given accident. So this first step guarantees the invocation of a subject. Well then, having this, it must be possible to switch over from the concrete term ' white ' to the abstract term ' whiteness ', and the Summa quotation in some sense legitimizes this.