Bound up in his adherence to the infinity of the divine nature (which is the only pure being), is Gregory’s commitment to divine simplicity. Eunomius was content to use the terminology of “simple,” however he held to a plurality of being among the Father, Son, and Spirit. Thus, for Eunomius, each being was itself simple, but each was also distinct from one another, allowing for a variation of quality between the various beings.
Nyssa rejects this “with all his might,” and explains that simplicity applies to the single divine nature, thus admitting no variance of quality between the persons sharing that being. Simplicity and infinity, both aspects of the same concept, are what allow for total equality between the persons of the godhead. Gregory states:
We believe that the most boorish and simple-minded would not deny that the Divine Nature, blessed and transcendent as it is, was ‘single.’ That which is viewless, formless, and sizeless, cannot be conceived of as multiform and composite. But it will be clear, upon the very slightest reflection, that this view of the supreme Being as ‘simple,’ however finely they [the Eunomians- sw.] may talk of it, is quite inconsistent with the system which they have elaborated. For who does not know that, to be exact, simplicity in the case of the Holy Trinity admits of no degrees. In this case there is no mixture or conflux of qualities to think of; we comprehend a potency without parts and composition; how then, and on what grounds, could any one perceive there any differences of less and more. For he who marks differences there must perforce think of an incidence of certain qualities in the subject. He must in fact have perceived differences in largeness and smallness therein, to have introduced this conception of quantity into the question: or he must posit abundance or diminution in the matter of goodness, strength, wisdom, or of anything else that can with reverence be associated with God: and neither way will he escape the idea of composition. Nothing which posses wisdom or power or any other good, not as an external gift, but rooted in its nature, can suffer diminution in it; so that if any one says that he detects Beings greater and smaller in the Divine Nature, he is unconsciously establishing a composite and heterogeneous Deity, and thinking of the Subject as one thing and the quality, to share in which constitutes as good that which was not so before, as another. If he had been thinking of a Being really single and absolutely one, identical with goodness rather than possessing it, he would not be able to count a greater and a less in it at all. It was said, moreover, above that good can be diminished by the presence of evil alone, and that where the nature is incapable of deteriorating, there is no limit conceived of to the goodness: the unlimited, in fact, is not such owing to any relation whatever, but considered in itself escapes limitation. It is, indeed, difficult to see how a reflecting mind can conceive on infinite to be greater or less than another infinite. So that if he acknowledges the Supreme Being to be ‘single’ and homogeneous, let him grant that it is bound up with this universal attribute of simplicity and infinitude. If, on the other hand, he divides and estranges the ‘Beings’ from each other, conceiving that of the Only-begotten as another than the Father’s, and that of the Spirit as another than the Only-begotten, with a ‘more’ and ‘less’ in each case, let him be exposed now as granting simplicity in appearance only to the Deity, but in reality proving the composite in Him.
~ Against Eunomius 1.19
Let us note that Gregory takes it for granted that the Supreme Being is one and simple. He relies on this axiom to show that simplicity entails an identification of nature, and thus the Only-begotten and the Spirit, being one with the Father, share his nature and all of his attributes completely. There can be no scale of greater or lesser among these without either severing the unity of nature or making the Supreme Being composite, which would also entail that it was not, in fact, the Supreme Being.
Furthermore, if there is such as thing as “absolute” divine simplicity (over and against other formulations of simplicity), and this is itself disputed and would be anachronistic to apply to the 4th century, Gregory would most certainly be affirming absolute divine simplicity. He states that a simple being is formless and sizeless. It admits of no difference of quality. The attributes of the divine nature are not external, but rather rooted in the very nature itself.
Finally, Gregory gives us the most straightforward affirmation of simplicity when he identifies the divine nature with the good. It does not merely possess goodness. It is identical with goodness. He identifies the essence of God with the attribute or property of goodness.
And so we have Gregory the Augustinian and Gregory the Thomist. We have Gregory the Western scholastic “essence-minded” theologian.