Scotus: haecceities must be some positive entity

Medieval philosophers offered a variety of theories about individuation -- that is, about the formal cause of individuation. Here are two popular medieval options.

(1) Some proposed that individuals are individuated by their unique set of incidental features. For instance, if we ask why you and I are different, the proponent of this theory would say that you and I have different heights, weights, shapes, colors, and so on, and this particular set of features that I have is unique: no other individual has these features, just as no other individual has your particular set of features.

This view obviously rests on the identity of indiscernibles: any things that have the exact same properties are indistinguishable, and hence the very same thing. So, for instance, if someone else looked exactly like me in all respects, you couldn't tell us apart, and for that reason we would be same individual.

One might object, of course, that we could easily distinguish me and my twin because I am sitting here, in this chair, and my twin is sitting there, in that chair. In other words, we occupy different regions of space-time.

But the advocate of the theory in question would say that space and time are also part of our incidental features, and so me and my twin would not have the very same features after all. We would have a lot of them in common, but we would not have our spatial and temporal features in common, and for that reason we would not be the same individuals.

Max Black famously questioned this theory. There's no reason, he proposes, that we couldn't imagine two exactly similar identical spheres in parallel dimensions. (I must admit that I have never understood this. Even if they have all the same features, they are still indexed to different dimensions, and so wouldn't they differ with respect to that? I confess that I don't get how Black's example is a genuine counter-example.)

(2) Others proposed that two individuals differ because they are made up of different chunks of materials. So, for instance, me and my twin might have all the same features, except I am made of this chunk of material, and he is made of that chunk of material.

(This theory seems to assume that different chunks of material are distinct -- either primitively, or in virtue of the fact that they occupy different regions of space. But again, couldn't one suppose: what if these two chunks of material had the same spatial and temporal properties? How would they be distinguished then? I think that most advocates of this theory would have to maintain that different chunks of material cannot occupy the same region of space-time simultaneously.)

Scotus argues that neither of these two theories actually explain individuation. For Scotus, each of these theories presupposes individual entities. The first theory assumes that incidental features are already individual, and the second theory assumes that chunks of material are individual. But that still leaves us wondering: how are incidental features or chunks of material individuated? So as Scotus sees it, neither of these theories have really explained individuation after all.

In fact, Scotus goes through all the major theories that he knows of and points out that they all presuppose some individual entity, and for that reason, he rejects all of them.

Scotus then concludes that we must assume that individuation comes about by some primitively individual entity -- and this is called a haecceity. We don't know what this entity is, but it must be a positively real thing, and it must be individual in and of itself. Since all of our theories about individuals assume individuality already, we must postulate some primitive individuator.