When a Thomist speaks of the “unified reality” that grounds the human capacities for thought, love, freedom, and bodily life, he is pointing to the substantial form of the living human being: the rational or intellective soul. In classical hylomorphism every material substance is a composite of matter (the principle of indeterminacy, the stuff that can receive different shapes) and form (the principle of determination, that which makes the stuff to be this particular kind of thing and equips it for its characteristic operations).
Alright. So "essence" is the "stuff" that causes something to be what it is. I'm gathering it is not really meaningful to talk about what makes up a particular essence, or essence in general. It's very abstract, and yet something real - you call an "underlying reality" and a "formative principle". And .....
In plants and animals the form is already called “soul,” because it is that by which the organism is alive at all; in man the soul is further specified as rational, since it founds powers that rise above the merely organic—universal intellection and free choice.
So the "soul" is the essence? At least with living things? And you would say a being "possesses" a human soul, and from that essence it will ultimately display the essential (from essence) traits that categorized the being as human?
DNA, neurons, hormonal cascades, and all the biochemical marvels we chart with microscopes belong to the material side of the composite. They are the structures and dispositions that the soul employs as instruments, the way a musician uses strings or pipes. Remove the soul, and the molecular patterns may linger for a while, but the coordinating principle is gone and the organism soon dissolves into a heap of tissues. Conversely, without that molecular scaffolding the soul cannot express its life outwardly: even the highest intellect needs a healthy cortex and sensory data to reason about justice or geometry.
Where does the concousness reside? With the essence?
Form and matter, therefore, interpenetrate without confusion: the soul is not a ghost floating in a machine, nor is it reducible to genetic code. It is the immanent cause that makes the bodily matter to be a this living, human body—one substance, not two loosely coupled agents.
So the “something there” is not an unknown x that we posit only to cover our ignorance; it is the formal cause whose existence we infer from evident effects.
Alright, and the formal cause, even when it imperfectly directs / causes a being, will tend to push the being toward what we would call a "nature" - which is identifiable by certain traits?
Whenever we see a genuinely human act—abstract thought, free consent, moral resolve—we know that a principle proportionate to such acts must be inside the agent, because nothing can give what it does not possess.
So would you say that we decern the presence of a human soul or essence by observing the nature - the outward expression of the essence as it directs, showing certain unique traits?