@MeanMrMustard
Your
instinct to test the vocabulary against concrete specimens is the right
one—Thomas would have applauded it—because in his view metaphysics must start ab
esse in re, from realities that can be named and handled, and only then
ascend to the heights. So let us keep our feet on the ground with dogs and
poems, and only at the end lift our eyes again to the mystery of God and of the
Incarnate Word.
When I said
“nature” is the set of essential properties while “essence” is the underlying
reality that grounds those properties, I was not offering a mere
re-labelling exercise. Think of a living oak. We can draw up a botanist’s
checklist—lignified tissue, photosynthesis, acorn reproduction—and that
description is certainly true of the oak. Yet the description is not what
makes the tree be a tree. Behind the checklist lies a formative principle that
actually organizes matter into roots, trunk, leaves, and powers of growth.
Aristotle called that principle the form, and when form organizes matter
into a complete, self-subsisting whole we call the result a substance.
The Thomistic tradition reserves the word essence for that formative
principle considered precisely as the source of intelligibility—that by
which the oak is an oak. Nature, in turn, names the ordered set of
capacities that flow from that essence: absorbing water, synthesizing sugars,
pushing out new rings each spring. Essence is therefore not a conceptual filing
cabinet but a real, interior cause; nature is that cause in act.
Nature isn’t just a tally of traits; it’s the dynamic principle that governs how a thing operates, flowing from its essence. Essence, meanwhile, isn’t merely the sum of those traits either—it’s the metaphysical core, the “whatness” that makes a thing intelligible as itself, distinct from everything else. Think of essence as the internal blueprint, and nature as how that blueprint plays out in reality. They’re not redundant; they’re two sides of a coin, with essence being the “what it is” and nature the “how it acts because of what it is.” A thing’s operations reveal its nature, which is grounded in its essence.
The same
distinction holds for us. A newborn, an adult philosopher, and a comatose
accident-victim all share the same essence, rational animality. In a
newborn the intellect and will are only radically present—as the rose is
present in the living root—but they need not be on public display for the
essence to be whole and operative. If the powers were absent even radically,
the child would not be a human being in the first place. Hence neither a severe
cognitive disability nor the sleeper’s inactivity nor prenatal immaturity robs
the subject of humanity. What matters is the intrinsic principle that
grounds the capacity, not the momentary performance statistics.
You’re right that “two legs” doesn’t define humanity; a legless person is no less human. Thomism would say that’s because legs are accidental, not essential. The essence of a human is “rational animal”—a composite of animality (life, sensation, motion) and rationality (intellect and will). This isn’t a category in the nominalist sense, where we just slap a label on a set of traits we observe. It’s a metaphysical reality: what makes a human human is this inherent capacity for reason, even if it’s not always exercised. A mentally handicapped person or a fetus doesn’t lose humanity because the essence—rational animality—remains intact as their defining principle, even if accidents like injury or development impede its full expression. “Actualizing” a nature doesn’t mean ticking off a checklist of displayed traits; it’s the living out of those essential capacities, whether fully realized or latent. A fetus has the intrinsic potential for reason, which is what counts, not its current performance.
Now for
dogs and cats. A dog’s accidents—number of legs, color of fur—can vary wildly
without touching its dog-essence. If you amputate a leg, you impair the animal
but do not transmogrify it into a cat. Why? Because the essence, the formative
core, remains. That is why Aristotle speaks of “that which acts, acts according
to what it is”: the canine essence elicits barking rather than meowing, pack
instincts rather than solitary ambush, even when the creature’s accidental
equipment is damaged. So yes, dogs have a nature: call it “caninity.” It’s the essence that makes a dog a dog—sentient, corporeal, with a specific mode of animal life distinct from, say, a cat’s “felinity.” Both are mammals, sure, but their essences differ in form: a dog’s nature inclines it to pack behavior and certain sensory aptitudes, while a cat’s leans toward solitary hunting and agility. A three-legged dog doesn’t cease to be a dog because its essence isn’t tied to leg count but to that deeper form—its “dogness”—which persists despite accidental variations like color or size. The distinction isn’t just taxonomic; it’s real, rooted in their being, not our naming conventions.
A poem
stands in striking contrast. Ink and paper have no inherent tendency to rhyme,
to scan in pentameter, or to conjure images of moonlit seas. Those features
arrive only when a mind imposes them from without. A poem therefore possesses
no self-subsisting nature the way a dog does; its “whatness” is accidental,
borrowed, unable to act or grow of itself. A poem isn’t a substance like a dog or a human—it’s an artifact, a product of human craft. Its “essence” isn’t intrinsic but extrinsic, imposed by the poet’s intellect onto matter (words, rhythm). Its nature is its formal principle as a poem—what makes it a poem rather than prose or gibberish—but it’s not a substantial nature. It’s an accidental form, dependent on a mind to shape it, unlike the intrinsic essence of a living thing that drives its operations from within. A poem’s “whatness” is real, but it’s tethered to human intention, not self-subsistent being. The Thomist places such items in the
category of artifacts, where the form is extrinsic and perishes the
moment the organizing intelligence or the supporting material is withdrawn.
Those
creaturely examples also expose the contingency of everything around us. An
oak, a dog, a man—all are composites in which “what it is” and “that it is”
remain distinct. I might never have existed; I could cease to exist tomorrow.
This composition of essence (potency) and existence (act) cries out for a first
reality in which the two are no longer separable, otherwise nothing contingent
would ever be. God, says Aquinas, is precisely ipsum esse subsistens—Being
itself, not merely a being. In Him, essence and existence are numerically one;
He is sheer actuality, with no unrealized capacities, no admixture of limit or
defect. That is what “pure act” (actus purus) seeks to capture. When we
predicate power, knowledge, or goodness of God we are not stacking independent
properties like stickers on a suitcase; we are naming distinct aspects of a
single, limitless act of being.
For us, “what we are” (essence) and “that we are” (existence) are separate; I’m a human, but I might not have existed. In God, they’re identical: His essence is to exist, pure actuality with no potentiality or limitation. His nature and essence align as one eternal act of being, not a set of traits we compile but the ground of all reality. Concrete example? Think of God as the “why” behind everything existing—He’s not a thing among things but the source that doesn’t need a source, unlike dogs, humans, or poems.
Your worry about “essence” being just a set of attributes misses this: in Thomism, it’s not a descriptive bucket but a principle of intelligibility. For humans, it’s not about listing “rational” and “animal” like ingredients; it’s the unified reality that grounds those capacities. For God, it’s not attributes like omnipotence tacked on—it’s His sheer is-ness that entails them. This isn’t vague; it’s the bedrock of a system where language tracks reality, not just our perceptions. Christology leans on this: “fully God” means the Son shares that divine essence completely; “fully human” means He assumes our essence completely, uniting both in one person without blurring them. It’s not sloganeering—it’s metaphysics with teeth, meant to hold up under scrutiny like yours.
So all this
bears directly on the Incarnation. When the eternal Word assumed a human nature,
He did not tack divine attributes onto a Galilean rabbi, nor did He dilute His
Godhead into some tertium quid. Rather, the one divine Hypostasis
took to Himself—united to His person—a complete human essence: body, soul,
intellect, will. Everything that makes God God and everything that makes man
man is present, intact, in the single subject who speaks from the manger, from
the mount, and from the cross. “Without confusion, without change, without
division, without separation,” Chalcedon insists, precisely because only such
an unconfused union secures real divinity and real humanity and therefore real
redemption.
If you
press me for an “example” of actus purus, I can only warn that every
finite analogy will limp. Yet an echo can help: picture white light cascading
through a prism. The colors—red to violet—are distinct participations; the
white is indivisibly all wavelengths at once. Each color expresses something
truly present in the source, yet the source transcends any single hue. Likewise,
creatures manifest limited reflections of being; God is unbounded Esse
itself, containing every perfection formally, not fragmentarily.
Thus, the
Thomist map: essence is the intrinsic formative principle, nature the ordered
dynamism that flows from it; accidents hover at the surface, mutable and
secondary. Dogs and cats manifest their essences by what they do; a poem
receives its ephemeral “form” from outside; a human embryo is already human
because the rational essence is present implicitly; God alone simply is.
On that scaffolding the confession “fully God, fully man” stands firm, not as
rhetoric but as metaphysical precision wedded to revealed fact.