“Essence” is a placeholder for our grasp of the fact that a thing T is “at the very least Kp”, where p is the exemplification of a certain range of properties P and dispositions D, and K is a genus of Ts which p. Having a scientific grasp of the essence of T, or E(T), entails being able to say that T “is a K which, unlike other Kp, p(T·K)”, where p(T·K) is the range of P and D which predictably and falsifiably distinguish T from other Ts under K.
One problem in discussing these issues with scientismatics and/or the philosophically maladroit, is that common parlance treats kinds in a nominalist sense (e.g. x is unique = x is one of a kind). In fact, however, the actuality of A’s being x requires that it is at least one of a kind.
We never immediately know an essence, since an essence is not knowable apart from its actual operations, that is apart from its act of existence. There is no thing “in itself”, since everything exists by having actual causal (or at least counterfactual) connections to everything else in the order of nature. Hence, the desire to know an essence with infinite precision is a desire to remove the empiriological realities of T and treat it as a formal abstraction. This is the great lesson of Gilson’s Being and Some Philosophers.
Critics of Aristhomism reify abstractions and then accuse A-T of having a bloated ontology. Specifically, it seems gip thinks of “essence” as some extra layer superimposed on a thing, such as the virtus dormitiva he’s so fond of citing. This is I’d course an erroneous imposition on A-T. Essence is not an additional layer of being that exists over and above a thing’s properties and dispositions as we try to know the thing. Essence is simply that in virtue of which it is true to say a thing’s properties and dispositions cohere AS a single operative thing. Essence is that which provides noetic assurance, as it were, that we are tracking closer to a thing’s ‘proper’, ‘act-ual’ place in nature by following its operations, instead of merely cataloguing a heap of accidents and phenomena which merely reflect our investigative tools and observational biases.