Aquinas, Post-Mortem Survival, and the Weak Supplementation Principle
The weak supplementation principle states that if x is a proper part of y, then there is some other proper part z of y that does not overlap. Consequently, if one affirms the WSP then one thinks there cannot be an entity with just one proper part.
At least Eleonore Stump thinks Aquinas did not hold the WSP, “A human person is not identical to his soul; rather, a human person is identical to a particular in the species rational animal. A particular of that sort is normally, naturally, constituted of an array of bodily parts and is composed of form and matter. Because constitution is not identify for Aquinas, however, a particular can exist with less than the normal, natural complement of constituents. It can, for example, exist when it is constituted only by one of its main metaphysical parts, namely the soul. And so although a person is not identical to his soul, the existence of the soul is sufficient for the existence of a person.” (Aquinas, pg. 53).1
This interpretation leads to the survivalist rather than corruptionist view of the post-mortem state, which has been discussed on the podcast with Gaven Kerr (survivalist) and Christopher Tomaszevski (corruptionist).
Another thought might be this: on Aquinas’s view, it might be the case that more than one proper part is necessary at some time (say, a human person’s their beginning) but not all times.
Though, technically, we still have composition on the essence/existence level, so maybe this doesn’t rule out WSP after all…