The word “subsistent” is tricky. Modern day dictionaries define “to subsist” as “to be sustained,” or “to maintain existence” or “to support or maintain with provisions or funds, to support.” This meaning is consistent with the word’s etymology. In classical Latin, subsistere meant “to stand firm” or “to come with relief or support.” In post-classical Latin the word meant “to exist as a substance,” or more simply, “to be.” This is how St. Thomas Aquinas used the word in the Summa Theologiæ (First Part, Question 75, Article 2). After he proved the soul is immaterial, he asked, “Whether the human soul is something subsistent (subsistens)?” Does the human soul exist as a substance, rather than emerge as some property of matter?
In daily life, we don’t spend a lot of time thinking about that question, but much of the gamut of modern physical and biological sciences assumes the soul does not exist. The unproven postulate that nothing immaterial can exist because science cannot measure or observe the immaterial pervades this scientific age. It is true, the science of material things is limited to what is material; but it is an error to stretch that assumption beyond science and to call that assumption dogmatic truth.
How did St. Thomas prove the subsistence of the soul? He cited St. Augustine’s fifth century treatise, On the Trinity (Book X, Chapter 7). Quoting:
“Now, in the case of all these opinions, any one who sees that the nature of the mind is at once substance, and yet not corporeal—that is, that it does not occupy a less extension of place with a less part of itself, and a greater with a greater—must needs see at the same time that they who are of opinion that it is corporeal do not err from defect of knowledge concerning mind, but because they associate with it qualities without which they are not able to conceive any nature at all.”
In other words, as long as we try to picture the soul, we will try to define it as a body accessible to our senses. We have to go beyond mental images (i.e. imagination) and launch our minds into the abstract (i.e. conception). Because we have to do that, we prove to ourselves that the soul, who knows itself, is not an emergent property derived from sensory chemicals interacting with the brain.
I’m trained as a scientist, and let me tell you, it has taken years for that to even begin to sink in. It was a stumbling block, as they say, for faith. Logical arguments do not satisfy me like hard data does because hard data has a way of imposing itself on the researcher. “Show me,” the incurable reductionist says, “put it there on the table, and let me analyze it for myself.” How do you put logic on the table? (Which is kind of the point.) St. Augustine and St. Thomas wrote out the reasoning for us to follow, if we let ourselves go there.
What they wrote, I think, comes down to a question we must ask ourselves. “Am I more than particles?” Because if my subsistent soul is the very life of my body beyond my senses, then my soul is immortal and the materialist world order is turned on its head. If I am just particles? I don’t know. I never could reconcile that assumption with what I know, deep in my heart (but not the material heart), to be true. The answer here is not just definitional—it cuts right to the truth of our existence as persons rather than machines. It goes straight to the question of faith beyond the material world.