Literally, homoousion means "same substance" from homos (same) and ousia (essence) (The Free Dictionary). But it is often translated as “one substance” (e.g., Merriam-Webster).
The word “same” has more than one possible meaning. An article on "Identity" in the Stanford Encyclopedia of Philosophy, states that a distinction is customarily drawn between qualitative and numerical sameness:
Things are qualitatively the same if they share properties. Therefore, things can have different levels of qualitative sameness. The article provides the following examples:
- Poodles and Great Danes are qualitatively the same because they share the property of being a dog, and such properties as go along with that.
- But two poodles will have greater qualitative sameness.
On the other hand, things are numerical the same if they have absolute, or total, qualitative sameness. In that case, the two things actually are one thing; therefore the term “numerical.”
So, two perfectly manufactured identical rubber balls on a production line are homoousion but still are two because they do not have total or complete qualitative sameness. They differ, for example, in terms of space occupied.
To apply this to the word "homoousion" in the Nicene Creed:
- “One substance” implies that the Father and Son share one single (numerically the same) substance while
- “Same substance” can mean both qualitatively or numerically the same substance. The question is, what did the authors of the creed mean?
Tertullian also described the Father and the Son as of the same substance. He wrote: "For the Father is the entire substance, but the Son is a derivation and portion of the whole" (Against Praxeas, Chapter 9).
In Tertullian, the substance of the Father and the Son are qualitatively the same. They are not numerically the same because the substance of the Son is only a portion of the substance of the Father.
I also know that during the years of intense controversy after the Nicene Creed, many concepts were developed that did not exist in the year 325. So, the question is, how would we know whether the Nicene Creed uses homoousion in the sense of numerical or qualitative sameness?