  Although relations of reason force a shift away from the Aristotelian characterization of relations, it is important to recall that they were originally invoked to preserve the deep-seated Aristotelian conviction that no substance is a relation. As we have seen, it was to avoid saying that Socrates is signified by the concept ‘thought about’, when he is being thought about by Simmias, that medieval philosophers invoked the notion of a relation of reason: it is not Socrates, but a relation of reason, they say, that corresponds to the concept in question, and in this way avoid the consequence that a substance such as Socrates is a relation. And of course it was the same sort of invocation that enabled them to avoid saying that Socrates corresponds to the concept of ‘self-identical’ or that God corresponds to the concepts ‘Lord’ or ‘Creator’. Given the lengths to which medieval philosophers are willing to go, in situations such as these, to preserve the Aristotelian thesis that no substance is a relation, it is all the more surprising that there is at least one case—namely, the Christian doctrine of the Trinity—in which they are forced to admit that even it cannot be upheld.