In a discussion in his newer introduction to medieval philosophy text, John Marenbon considers the following principle in Aquinas, which he calls 'Aquinas's Principle':
If the antecedent of a conditional contains a cognitive proposition, the consequent should be understood according to the mode of the knower, and not that of the thing known.
The illustration is the conditional,
If I understand something, it is immaterial.
This, if 'Aquinas's Principle' is used, should be understood as:
If understand something, it is immaterial according to its being understood.
To this Marenbon replies that there are plenty of conditionals with cognitive antecedents to which Aquinas's Principle does not apply. His example:
If I feel something with my touch, it is a material thing.
Of which he says, "The consequent...does not need to be qualified by a phrase such as 'according to its being touched'. It is simply true that anything I can touch must be material."
Whatever may be said of 'Aquinas's Principle', Marenbon's counterexample seems to me to be badly chosen. For while it may be simply true that anything I can touch must be material, it does not follow that anything I touch must be simply material. The natural way to understand Marenbon's conditional is to understand it as meaning,
If I feel something with my touch, it is a material thing insofar as it is touched.
For instance, there are plenty of entities that can be touched but are not simply material; for example, a university, which is material to the extent that you can touch it, but is not insofar as it is (for instance) a legal corporation. And the qualification could still be added, without significant change of meaning, for purely material things -- it's just that there would be very little point in doing so. The fact that we don't need it for practical purposes isn't an adequate reason for rejecting 'Aquinas's Principle'.
In any case, I don't see any reason to hold that Aquinas held 'Aquinas's Principle' in an unqualified way; it seems to me that Marenbon takes Aquinas's words out of context and interprets them out of that context. The natural way to read Aquinas's statement in context is to take him as saying that when the antecedent clarifies that the existence in question is existence in the soul rather than in itself, we must not then take the consequent as saying anything about existence in itself. The principle that is really doing work here is not the claim about conditionals, however, but the claim that "the existence of a thing in itself is different from the existence of a thing in the soul."
***
John Marenbon, Medieval Philosophy Routledge (New York: 2007) 253-254.
