do these two thinkers say who have decisively determined all Western speech and thought about the essence of things?
Reviewing the rest of Aristotle’s characterizations of the essence, we come upon a determination that is so simple it says nothing to us: the essence is what we seek when we ask τί ἐστιν: what is this? What is this here and that there? A plant, a house. The essence is the τί εἶναι—the whatness [Wassein] of a being. To ask what something is is all too familiar to us and to earlier generations. What something is is its essence. But what is this “what” itself? Is there an answer? To be sure. Plato provided it. What something is, the whatness (τὸ τί εἶναι), e.g., of a house or a man, is what is constantly present in that something. In all ever so different houses what is constant is what they are, “house,” and conversely, what they are, houses in all their variety and change, is the constant. A house could not collapse if it were not a house.
This constant presence is what we have in view in advance, though without considering it explicitly, when we name and experience whatever we encounter as what it is, e.g., as a house. When we enter a house we pay attention to the door, the staircase, the halls, and the rooms, and only to these, for otherwise we could not move around in it at all. On the other hand, we do not pay attention explicitly and in the same way to what all that is in its unity, namely house. Nevertheless, precisely what it is, house, the essence, is always sighted in advance, though not explicitly considered. In fact, if we did engage in such a consideration of the essence we would never come to enter the house and live in it. Nevertheless, again, what the thing is, the constantly present, must be sighted in advance and indeed necessarily so. “To see” is in Greek ἰδεῖν; what is in sight, precisely as sighted, is ἰδέα. What is sighted is what the being is in advance and constantly. The “what it is,” the whatness, is the ἰδέα; and conversely, the “idea” is the whatness, and the latter is the essence. More precisely, and more in the Greek vein, the ἰδέα is the look something offers, the aspect it has and, as it were, shows of itself, the εἶδος. Only in light of what is seen in advance and constantly, yet not explicitly observed, e.g., house, can we experience and use this door as a door, this staircase as a staircase to this storey with these rooms. If that were not in sight, how would matters then stand? You may think that out for yourselves.