What did the fathers who formulated the Nicene Creed mean to assert when they described the relationship between Father and Son with the term homoouios, “same substance”? In what sense “substance” and in what sense “same”?
Surveying the usage of this terminology before Nicea, Christopher Stead (Divine Substance) suggests several possibilities.
First, homoousios might mean “that the so-called two or more things are actually one and the same; the reference to ‘substance’ merely advises us that in calling them ‘two’ or ‘three’ or some other number, we are attending to distinctions which are not those of substance; for instance, we are recognizing different names, functions, or contexts of what is really one individual thing” (246).
This interpretation takes ousia as Aristotle’s “primary substance,” that is, substance as an individual thing. But for several reasons it’s by no means certain that this is what the creed means. “Aristotle employs this concept, as traditionally understood, only in his Categories, and here only of embodied creatures like ‘this man,’ ‘this horse’; he did not hold that immaterial realities are individuated in the same way, since . . . he thought that things are individuated by their ‘matter'” (246).
Besides, “unless we are dealing with philosophers professedly discussing Aristotle, it is not easy to be sure whether this concept is really intended; since ancient usage allows it to be said that two things are ‘identical as regards their substance’ when all that is meant is that they are members of the same genus or species; and we have also noted the flexibility of modern usage in speaking of ‘one thing’, and the complexities presented by individual species” (246).
Second, “Two or more beings could be called homoousios because there is one single ousia to which they belong, and of which they are aspects, parts, or expressions; and since ‘aspects’ and ‘expressions,’ no less than ‘parts,’ imply that other aspects, expressions, or parts are possible, there is a sense in which the ousia referred to contains more than any one of its aspects, expressions, or parts. Thus three divine Persons might be described by the term homoousios as belonging to a single complex ousia which needs the three distinct Persons for its full expression” (246-7).
Finally, “the term homoousios could be applied to two or more beings because they severally have (and not ‘jointly constitute’) a single ousia; that is, if they have the same generic or specific characteristics, or the same material constitution. Most probably ancient writers do not draw the line between this third possibility and the second in the way which would seem natural to us; since to have the characters which identify a species is to belong to that species; and the ancients often refer to the members of a class, species, or genus as ‘parts’ of it; they also quite commonly attribute to classes, for instance to animal species, a metaphysical status influenced by Platonic theory which we might refuse to concede. Nevertheless it would be natural to think that the notion of ‘belonging to an inclusive reality’ would be easily suggested where one could claim for this reality some natural unity and structure, or again in the case of a class with a limited number of members; where these conditions are absent, it would be more natural to think in terms of ‘sharing some identical feature.” Thus the term homoousios, used of angels, might suggest that they all belonged to the same glorious company; used of stones, it might rather suggest that they share those features which inseparably attach to stones, in being inanimate, heavy, and hard” (247).
He cites an example from Irenaeus who calls the Aeons of Valentinus homoousioi because “he is thinking of their common constitution as pneumatikoi, beings activated by spirit, rather than of the complex body, the Pleroma, to which they belong” (247).
Judged by its pre-Nicene usage, “Homoousios guarantees very little; it can be used of things which resemble one another merely in belonging to the created order, or to the category of substance; it can relate collaterals to each other, or derivatives with their source; it does not exclude inequality of status or power” (247).
Yet, it’s far from meaningless: “the term is often used to indicate a relationship which in fact is closer than mere membership of the same species or similar material constitution, for instance that of a stream to the actual fountain from which it flows, or that of an offspring to his own parent. To call a son homoousios with his father implies more than merely their common membership of the human race; and the further implication need not be merely that of their physical linkage; the term can evoke their whole biological and social relationship” (247-8).
Stead concludes “all (or virtually all) the theological applications of it in pre-Nicene times fall within this elastic third class; we have found no evidence which clearly points to the first or second” (248).
He goes on to raise the question whether pro-Nicene used the term in the first or second sense. There are some turns of phrase (in Marcellus and Constantine) that suggest an equation of ousia with primary substance, but he doesn’t think the evidence strong.
He also doesn’t find much evidence for the second usage. Did some think, he asks, “the Triad of persons is called homoousios because they jointly constitute the single complex reality which is the divine Monad?” He finds this implausible because “this scheme is seldom presented in a pure form. The pure form would be a symmetrical scheme in which all three persons play an equal part. What in fact we find is that the divine Monad and source of the Triad is identified with the first person, the Father” (249).
David Brown (Divine Trinity, 241) concludes from Stead’s argument that “Nicaea leaves open the question of the nature of the divine unity, whether it is numerical or generic, but that, if anything, its bias seems to be in the latter direction.”