Ask a Thomist

How is Jesus ‘Consubstantial’ With the Father?

If God does not come under the genus of a substance, then why is Jesus the Son of God, ‘consubstantial’ with the Father? – John

Excellent question! St. Thomas asks the question of whether God pertains to any genus in the Summa Theologiae Ia Q. 3, a. 5. He gives many reasons why God cannot. The second reason he gives is most pertinent for our purposes.

It follows from the fact that the being (esse) or act of existing (actus essendi) of God just is his essence. So, if God were in some genus, it would have to be the genus of being itself. But, as Aristotle taught, being cannot be a genus, because every genus has differences which are “outside” the essence of the genus, but no conceivable difference falls “outside” being.

Continuing on in our understanding of substance, we know that what comes under the genus of substance gives partial or particular realization to what it means to be a substance. To speak in terms of the metaphysics of creation, we say that all substances are but a partial or particular realization of the many ways that creation can participate the inexhaustible being of God. Now, it’s also true that the relation between God and creation is non-reciprocal. God doesn’t come under the genus of substance; rather, he transcends it and gives the very act of existence to all that falls within it (speaking here metaphysically and not merely logically).

And yet, there is some intelligible connection between what we commonly refer to as substance (rocks, trees, dogs, men) and God. Let’s begin with the logical definition of primary substance given by Aristotle: a substance names what is neither said of nor present in anything. Rather other things (accidents) are said of and present in substance. This is partially true of God in a logical sense, that is to say, in matters of predication. God is neither said of nor present in anything else in matters of predication. It is also partially untrue, for nothing is said of or present in God, because he is simple, that is, utterly uncomposed.

Let’s switch from a logical to a metaphysical approach. In this way, it is also partially true to say that God is a substance. In the metaphysical sense, a substance refers to an individual being which subsists in a nature. Now, while this understanding of substance applies to rocks, trees, dogs, and men, it is also true of God that he is possessed of a metaphysical unity and that he subsists in a nature, so God can be said to be substance in a sense. On the other hand, God is not a substance in the way that other substances are (composed of matter and form with a circumscribed nature limiting esse).

So I think now we have a sense for 1) Why God does not belong to a genus in the strict sense and 2) Why God can be spoken of in terms of being or substance in an analogical and extended sense.

All that remains now is a little grammar. When we profess the Son to be consubstantial with the Father, that word translates “consubstantialem” in Latin which translates “homoousios” in Greek. The word basically means one in being or one in substance. God is one substance or being in that he is one divine existent. There is only one God–one divine nature. Each of the three Persons of the Trinity subsists in that divine nature, but distinctly by virtue of their respective origins. The Father subsists in the divine nature as unbegotten. The Son subsists in the divine nature as begotten of the Father. The Holy Spirit subsists in the divine nature as proceeding from Father and Son. Each Person is God and is everything proper to the divine essence (nature, substance). Thus, the Persons of the Trinity can be spoken of as consubstantial while God does not fall within the genus of substance.


Fr. Gregory Pine, O.P. serves as the Assistant Director for Campus Outreach for the Thomistic Institute. Born and raised near Philadelphia, PA, he later attended the Franciscan University of Steubenville, studying mathematics and humanities. Upon graduating, he entered the Dominican Province of St. Joseph in 2010 and was ordained in 2016. “It was St. Thomas Aquinas who first introduced me to the Order, and by his prayers that I grew in knowledge and love of its saving mission and ultimately came to find my happiness in Order of Friars Preachers.”

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