  [3] Sometimes, however, a relation is something real in one of the relata and a mere being of reason in the other. And this happens whenever the two relata do not belong to a single order. For example, sense perception and knowledge are related to things that are sensible and intelligible. But insofar as the latter are things existing in extramental reality, they are outside the order of sensible and intelligible being. And so there is a real relation in the knowledge and sense perception in virtue of the fact that they are [really] ordered to things that can be known or sensed. However, things [that can be known or sensed] are outside this sort of order [when] considered in themselves, and hence there is not really a relation in them to knowledge or sense perception. On the contrary, there is only a relation in them according to reason, insofar as the intellect apprehends them as terms of the relations of knowledge and sense perception. This is why the Philosopher says in Metaphysics V that they are spoken of relatively, not because they are related to other things, but because other things are related to them. Similarly, being to the right is not said of a column unless it is placed to the right of some animal. Hence a relation of this sort is not really in the column but in the animal. (Summa Theologiae I, q. 13, a. 7, corpus.)