In discussions of trinitarian doctrine, its commonplace for people to want to distinguish between “generic unity” and “numerical unity” when talking about consubstantiality. What is meant by “generic unity” is that the persons of the Trinity share in a common essence, meaning that the persons share the same divine nature or genus. This is frequently contrasted with “numerical unity”, the meaning of which tends to vary some. Sometimes, this boils down to describing the persons of the Trinity as a single individual, a single person. In other instances, this is used to try to distinguish between the idea that the persons are of the “same essence” from the idea that They are “one essence”.
This language is somewhat lamentable, as the term “numerical unity” is quite vague, and according to what it sounds like, could just as well be used to refer to generic unity as well, since we can just as well count natures as we can count individuals. The Trinity, of course, is not a single individual, and in cases that this is what is intended by “numerical unity”, it is tantamount to modalism. What I would like to address in this post, however, is the notion that there is a difference between “one essence” and “same essence”.
The short answer is, there is no difference. It is two ways of saying the exact same thing; thus the Nicene Creed, which employs the word ‘homoousias’ (literally homo=same, ousia=essence, ‘same essence’) sees this word translated both ways, but more commonly as “one essence”.
This is an important point, because often, when a distinction is drawn between these two expressions, “one essence” is ultimately getting used in a way that is modalistic. This meaning of “one essence”, as a redefinition of ‘homoousias’ by later theologians contrary to its intended meaning, has been treated in The Grievous Error of the Fourth Lateran Council.
What, then, is the difference? The Fourth Lateran council, and many others, intend to indicate a single individual reality, or person, by “one essence”; whereas the fathers who introduced the language of ‘homoousias’ intended the language to instead signify what gets labeled “generic unity”, that is, that the persons of the Trinity share the same divine nature (contra Arianism).
This idea can just as well be summed up by “one essence” as it can be by “same essence”. This is because when two things are entirely identical, with no difference that distinguished them, either in nature, or subsistence, or body, or time, or space, or any other way that two things are distinguished as being distinct from one another, they cannot rightly be counted as “two”, but as one. In the case of the divine nature, this is precisely what we are dealing with; the nature shared by all three persons is identical in each person, without variation. As a “nature”, or “genus”, then, there is nothing on account of which we could count the nature to anything beyond one. For the persons, then, to share the same nature, is for Them to have one nature, or one essence.
It is noteworthy that the fathers who introduced the language of ‘homoousias’ defined it in terms of this sort of generic unity, a sameness and identicality of nature among the persons of the Trinity. For example, Athanasius said:
“Even this is sufficient to dissuade you from blaming those who have said that the Son was coessential with the Father, and yet let us examine the very term ‘Coessential,’ in itself, by way of seeing whether we ought to use it at all, and whether it be a proper term, and is suitable to apply to the Son. For you know yourselves, and no one can dispute it, that Like is not predicated of essence, but of habits, and qualities; for in the case of essences we speak, not of likeness, but of identity. Man, for instance, is said to be like man, not in essence, but according to habit and character; for in essence men are of one nature. And again, man is not said to be unlike dog, but to be of different nature. Accordingly while the former [men] are of one nature and coessential, the latter are different in both.”
It is significant here that he employs the analogy of three men to define what he means (and he is giving a definition of the word ‘homoousias’ here). Those who would see a difference between “same essence” and “one essence” would be willing to say that men are of the same essence, but not “one essence” as the persons of the Trinity are. Yet, we can see that no such distinction was drawn by those responsible for introducing the language of co-essentiality into trinitarian dogma in the first place.
Also noteworthy is Hilary of Poitiers’s definition of “essence” given in De Synodis:
“Since, however, we have frequently to mention the words essence and substance, we must determine the meaning of essence, lest in discussing facts we prove ignorant of the signification of our words. Essence is a reality which is, or the reality of those things from which it is, and which subsists inasmuch as it is permanent. Now we can speak of the essence, or nature, or genus, or substance of anything. And the strict reason why the word essence is employed is because it is always. But this is identical with substance, because a thing which is, necessarily subsists in itself, and whatever thus subsists possesses unquestionably a permanent genus, nature or substance. When, therefore, we say that essence signifies nature, or genus, or substance, we mean the essence of that thing which permanently exists in the nature, genus, or substance.”
Hilary of known as “the Athanasius of the West” and “the hammer of the Arians”; and we see him define co-essentiality in the same way Athanasius did, as teaching that the persons of the Trinity share the same nature or “genus”. For him, “one essence” and “same essence” are the same thing.
Also noteworthy is his admission that ‘homoousias’ and ‘homoiousias’ mean the same thing when each is understood orthodoxly; something those who hold to a later re-definition of ‘homoousias’/’one essence’ are unable to say:
“Holy brethren, I understand by ὁμοούσιον God of God, not of an essence that is unlike, not divided but born, and that the Son has a birth which is unique, of the substance of the unborn God, that He is begotten yet co-eternal and wholly like the Father. I believed this before I knew the word ὁμοούσιον but it greatly helped my belief. Why do you condemn my faith when I express it by ὁμοούσιον while you cannot disapprove it when expressed by ὁμοιούσιον? For you condemn my faith, or rather your own, when you condemn its verbal equivalent. Do others misunderstand it? Let us join in condemning the misunderstanding, but not deprive our faith of its security. Do you think we must subscribe to the Samosatene Council to prevent any one from using ὁμοούσιον in the sense of Paul of Samosata? Then let us also subscribe to the Council of Nicæa, so that the Arians may not impugn the word. Have we to fear that ὁμοιούσιον does not imply the same belief as ὁμοούσιον? Let us decree that there is no difference between being of one or of a similar substance.”
Finally, a quote from Basil the Great, a post-nicene father from the following generation:
“The distinction between οὐσία [essence] and ὑπόστασις [person] is the same as that between the general and the particular ; as, for instance, between the animal and the particular man.” (Letter 236)”
“Suppose then that two or more are set together, as, for instance, Paul, Silvanus, and Timothy, and that an enquiry is made into the essence or substance of humanity; no one will give one definition of essence or substance in the case of Paul, a second in that of Silvanus, and a third in that of Timothy; but the same words which have been employed in setting forth the essence or substance of Paul will apply to the others also. Those who are described by the same definition of essence or substance are of the same essence or substance when the enquirer has learned what is common, and turns his attention to the differentiating properties whereby one is distinguished from another, the definition by which each is known will no longer tally in all particulars with the definition of another, even though in some points it be found to agree.” (Letter 38)
Here we see again, that ‘homoousias’ was meant by the fathers who promoted it as indicating that the persons of the Trinity share the same divine nature, comparable to how three men share the same human nature.
Finally, it is noteworthy that not only did the fathers who promoted the ‘homoousian’ language not intend it to signify something other than “generic unity”, but they actually rejected other possible definitions of the term “homoousias” that approach what many since have wanted to distinguish as “numerical unity”, that the three persons are in some way a single individual:
“Many of us, beloved brethren, declare the substance of the Father and the Son to be one in such a spirit that I consider the statement to be quite as much wrong as right. The expression contains both a conscientious conviction and the opportunity for delusion. If we assert the one substance, understanding it to mean the likeness of natural qualities and such a likeness as includes not only the species but the genus, we assert it in a truly religious spirit, provided we believe that the one substance signifies such a similitude of qualities that the unity is not the unity of a monad but of equals. By equality I mean exact similarity so that the likeness may be called an equality, provided that the equality imply unity because it implies an equal pair, and that the unity which implies an equal pair be not wrested to mean a single Person. Therefore the one substance will be asserted piously if it does not abolish the subsistent personality or divide the one substance into two, for their substance by the true character of the Son’s birth and by their natural likeness is so free from difference that it is called one.
68. But if we attribute one substance to the Father and the Son to teach that there is a solitary personal existence although denoted by two titles: then though we confess the Son with our lips we do not keep Him in our hearts, since in confessing one substance we then really say that the Father and the Son constitute one undifferentiated Person. Nay, there immediately arises an opportunity for the erroneous belief that the Father is divided, and that He cut off a portion of Himself to be His Son. That is what the heretics mean when they say the substance is one: and the terminology of our good confession so gratifies them that it aids heresy when the word ὁμοούσιος is left by itself, undefined and ambiguous. There is also a third error. When the Father and the Son are said to be of one substance this is thought to imply a prior substance, which the two equal Persons both possess. Consequently the word implies three things, one original substance and two Persons, who are as it were fellow-heirs of this one substance. For as two fellow-heirs are two, and the heritage of which they are fellow-heirs is anterior to them, so the two equal Persons might appear to be sharers in one anterior substance. The assertion of the one substance of the Father and the Son signifies either that there is one Person who has two titles, or one divided substance that has made two imperfect substances, or that there is a third prior substance which has been usurped and assumed by two and which is called one because it was one before it was severed into two. Where then is there room for the Son’s birth? Where is the Father or the Son, if these names are explained not by the birth of the divine nature but a severing or sharing of one anterior substance?
69. Therefore amid the numerous dangers which threaten the faith, brevity of words must be employed sparingly, lest what is piously meant be thought to be impiously expressed, and a word be judged guilty of occasioning heresy when it has been used in conscientious and unsuspecting innocence. A Catholic about to state that the substance of the Father and the Son is one, must not begin at that point: nor hold this word all important as though true faith did not exist where the word was not used.” (Hilary of Poitiers, De Synodis)