  The Trinity caused particular problems for logicians because of the unique relation between the three persons and the one divine essence which is, as Ockham put it in his Summa logicae, “one and simple and one in number and singular to the highest degree, and yet is several things.” Such inferences as “This divine essence is the Father, this divine essence is the Son, therefore the Father is the Son” seem to have true premises which assert an identity between singular things, yet the conclusion is unacceptable. Various solutions were found, including an exploration of different senses of identity and of different types of predication. In his Perutilis logica Albert of Saxony suggested that in expository syllogisms the middle term should always be qualified by the phrase, “Everything which is,” which makes the premises false in the divine case. In the Summa logicae, Ockham preferred to say that a singular proposition of the sort required for expository syllogisms could not have a subject which supposited for a reality that was both one and many, as occurs in the Trinitarian example.