After writing “Did the Incarnation Cause God to Change?”, I received an excellent follow-up question. Wrestling with my article, one of our readers could not get past how we can say that Christ was truly and fully man and yet not a “human person.” Could we say that somehow, Christ could be both a human person and a divine person, in the sense of his human person being a “partaker” in the divine person?
In short, the answer is no. There is only one person, or subject, in Christ, and that person is God! A divine person cannot change into a “divine-human” mix.
To understand why, we must define three essential terms, without which any explication would be futile: person, substance, and nature.
The classic definition of person comes from the great sixth-century philosopher Boethius, who defined it as “an individual substance of a rational nature.”
There is an essential term there that itself needs defining: substance. In philosophy, substance corresponds to the Greek ousia, or “being.” It is defined as that which undergirds, grounds, or (even better) constitutes an individual thing and does not inhere in anything else. It is not a property of a thing, but the thing itself.
There are literally billions of examples of substances we could choose to talk about, such as a “tree,” “horse,” “man,” etc. But we’ll consider “man,” since the man Jesus Christ is central to our undertaking here.
So what constitutes the substance of man? What is most basic to man, without which you do not have a man at all? The answer is the body-soul composite. Without either a body or a soul, you don’t have the “substance” of a man. You don’t have a man at all in the fullest sense.
Now, if we could move briefly back to the idea of person, we can see how substance is closely related to it, because both refer to the “subject.” However, there are crucial differences as well. Person and substance are different, because although all persons are substances, not all substances are persons. A “horse” is a substance but not a person, because a person is “an individual substance of a rational nature.” A horse does not possess a rational nature.
This takes us to our third essential definition. The simple definition of nature is the what of a thing—in contrast to person and substance, which refer to the individual subject being considered. In the case of rational beings, we are speaking of the what rather than the who.
Nature and substance are almost synonyms, but not quite. Because remember: substance, though similar to nature in referring to what constitutes a thing, also refers to the subject as well. In the case of rational substances, that would be the who of a being. Nature does not. It always refers to “what” a thing is.
Now that we have our three definitions, consider that all living human beings are persons, but person is not part of the definition of what it means to be fully human. The definition of what a human being is pertains to the what or the nature of a human being. So there is no reason why we couldn’t have a being that was truly human, or, more accurately, possessed a human nature but was not a human person. There is nothing in the definition of a human (being a body-soul composite) that requires it to be a human person. Thus, even though this actually happens only in the case of Christ, there is nothing unreasonable about positing the possibility.
(One important addendum: We know now that all living human beings are also human persons from the moment of conception, but Catholic thinkers once disagreed over when a human becomes a person, due to the faulty notion of “ensoulment” that was the common scientific understanding of prenatal human development from the ancient Greeks until relatively recently in history.)
Some questions arise. For example, what is the difference between substance and person when referring to human beings? They both seem to pertain to the who or the subject being considered.
Substance does always refer to the subject being discussed. True enough. And the same can be said of person. But remember, not all substances are necessarily persons. A person must be a rational substance, not just a substance. Angels and humans are the only created substances we know of in the universe that are both substances and persons. But there are billions and billions of substances that are not persons. And according to the revelation we have been given in Scripture and Tradition, Jesus is an example of a divine person who possesses a fully human nature but is not a human person.
Another question: Are the souls in heaven right now who do not have bodies human substances or persons? The answer is that they are neither. They are not fully human substances (though they do have substantial being), nor are they persons, because they do not have their bodies. Indeed, according to St. Thomas Aquinas, they are not fully human because it is essential to human nature to possess a body (Summa Theologica, III, Q. 50).
Some philosophers refer to the blessed in heaven, as long as they lack their bodies, as incomplete substances. This may be why texts like Revelation 6:9 and Hebrews 12:24 refer to those deceased in Jesus Christ as “souls” and “spirits.” The Church does the same. This is why we Catholics refer to the “souls” of our loved ones in the Church suffering (purgatory). Even though these souls are fully conscious and are the same “selves” they were on earth, they are not fully persons until the resurrection of the body.
With all of this acquired knowledge, perhaps we can more readily see just how Jesus Christ could reasonably have two natures, one human and one divine (revealing “what” he is), subsisting within one subject, substance, or person (revealing “who” he is) who is God. The possession of two natures in one person occurs only in Christ, just as the possession of a human nature without a human person occurs only in Christ. But the point is, this is both biblical and entirely reasonable.