Individuation is a question of the formal cause

When scholastic philosophers discuss individuation, their basic question is this: which of a thing's constituents make it the individual that it is? This question deserves some comment. As the schoolmen see it, we are looking for what they call the 'formal cause', not the 'efficient cause'.

The efficient cause is the agent that brings the effect in question into being. For instance, when a sculptor makes a statue, the sculptor is the efficient cause here, for the sculptor is the person/agent who effectively brings the statue into being. In short, the efficient cause is the producer of the effect.

The formal cause, on the other hand, is some feature or constituent of the product itself that explains why it is the sort of thing it is. For instance, the formal cause of a statue being a statue is the shape of the statue. Without it's shape, it wouldn't be a statue, so that very shape is the 'formal cause' of the statue being a statue.

When the schoolmen talk about individuation, everybody agrees that the efficient cause of the individual is its producer. But that seems perfectly obvious. If you want to know who produced this particular statue, the answer is the sculptor who actually produced it.

But when it comes to the formal cause, the schoolmen disagree. Again, here they are looking for some feature or constituent (or combination thereof) in the individual itself that explains why it is the individual it is.

More precisely: here we are looking for some set of features or constituents that cannot exist in some other individual. Suppose I ask every member of a group to take a side on capital punishment. Some will be for it, others will be against it, but in this case, taking the 'for' or 'against' side will separate these individuals into distinct groups, for nobody can be 'for' and 'against' capital punishment at the same time.

Taking the 'for' or 'against' side here would be an example of something that is the 'formal cause' of distinction: it's the sort of thing that cannot exist in more than one individual at the same time, and so when individuals take the 'for' or 'against' side of capital punishment, they necessarily get separated into groups. So also when it comes to individuals. The schoolmen are looking for some feature that cannot exist in more than one individual at a time, much like how being 'for' or 'against' capital punishment cannot exist in one and the same individual at the same time.