Alvin Plantinga’s response to the classic doctrine of divine simplicity is seminal in contemporary philosophy of religion literature. The basic argument is that God cannot be identical with his properties, because properties are impersonal, and God is a person. Siobhan Nash-Marshall (pdf) elaborates more on Plantinga’s argument here:
Plantinga takes properties necessarily to be univocal and invariant features of reality. This can be seen not just in his first objection to the doctrine of divine simplicity, the “substantial difficulty,” in which he claims that it is an “obvious fact” that God has several properties, since “he has both power and mercifulness … neither of which is identical to the other.” It is also plain in the third objection when he claims that it is “obviously absurd” to claim that two properties can be identical.
Now, God’s being just can only be necessarily distinct from his being merciful (as Plantinga claims they must), if only if justice and mercy are necessarily univocal and invariant features of reality: that is, if the properties “just” and “merciful” must exist in the same way in all of those substances, individual things, or non-abstract things in which they are instantiated. If it is necessarily true that “power and mercifulness … [are never] identical.” For if properties were not necessarily univocal and invariant features of reality—if they could exist in different ways in the different non-abstract individual things in which they are instantiated—then it would by no means be said that two properties which can in some instantiations be distinct properties, are necessarily distinct properties in all of their instantiations. [p. 9]
In response, she notes that the presuppositions of Plantinga’s argument would be utterly rejected by those theologians who held to the doctrine, using Anselm in particular as an example:
Saint Anselm’s response to this question would have been an unequivocal no. In Monologion XVI he indicates that properties can, and must in the case of man and God, vary in accordance with their instantiation [that is, ~ (p – 3)]. Man, he claims in that chapter, can be just, and God is unquestionably just, but this fact, he would have thought, does not entail that “justice” as it is instantiated in man and “justice” as it is instantiated in God are one and the same invariant property. Indeed, man’s “just” is nothing but his “possessing justice:” “For since a man cannot be justice but can have justice, a just man is not understood to be a man who is justice but to be a man who has justice.”35 God’s “being just,” on the other hand, is nothing but his being identical to justice: “So since the Supreme Nature is not properly said to have justice but rather to be justice, then when [this Nature] is said to be just, it is properly understood to be [a Nature] which is justice rather than to be [a Nature] which has justice.”36 Thus, were “justice” as it is instantiated in man and God to be one and the same property, man’s instantiating “justice” would be his instantiating Godness. This would not just have sounded absurd to Saint Anselm. He would have thought it blasphemous.
As the argument indicates, the belief that properties are invariant features of reality is not the only one of Plantinga’s beliefs concerning properties which Saint Anselm rejects. In Monologion XVI, as he had previously in Monologion VI, Saint Anselm flatly denies (p – 2) that properties are logically and ontologically prior to their instantiations. “Justice,” he claims, might perhaps not be quid sit summa natura [“what God is”], but a property distinct from God. After all, one might think that “for whatever is just is just through justice.”37 This would, of course, entail that “[t]herefore, the Supreme Nature is just only through justice.”38 But this thought, Anselm quickly adds: “But this [view] is contrary to the truth which we have already seen: viz., that—whether good or great or existing—what [the Supreme Nature] is, it is completely through itself and not through something other [than itself].39 Thus, he concludes, justice cannot be a property distinct from God. That is, it cannot be a property which is logically and ontologically prior to God. [pp. 11-12]
The doctrine of divine simplicity is easily one of the most misunderstood and maligned positions in discussions of the doctrine of God today. One cannot help but think that a little charity applied to thinkers of the past might have made it clear that simplicity could not be as stupid as many contemporary philosophers have made it out to be.