past, present, or future conditions to include all possible conditions. This is because the essence of a thing is not picked out by a mere empirical regularity but must also be maintained in the face of counterfactual situations. If I were to claim that (part of) the essence of a table is to be a wooden item of furniture, for instance, it would not establish this claim to merely show that all past, current, and future tables are wooden items of furniture (even if I could, in point of fact, be certain that there is not, never had been, and never would be such a plastic object). It would, in addition, have to be the case that a plastic object with exactly the same shape, resistance, function, and so on would not be a table. This means that for essential definitions, correspondence to the facts is a necessary but not sufficient condition for their being true.
Second, facts come too late for essential definitions, since we need to assume that the definition is true in order definitively to identify the fact or facts to which it corresponds. To get a feel for this, compare two other essential definitions, this time for gold:
(C) Gold is the noblest of the metals
(D) Gold is an element with atomic number 79.
When it comes to definitively founding simple factual statements like (A), we begin by finding the fact to which it corresponds, and we can do this by first finding the object referred to in the subject phrase – the lights – and then checking their condition. How about (C)? It seems like we would start by locating the object referred to in the subject phrase – gold. In fact, if (C) is an essential definition, the only way we can determine that gold is the noblest of the metals is by first finding some gold, and we do this by looking for instances of the noblest metal. Thus we see that in order to establish the truth of the essential specification, we first have to assume that it is true. And that means that we are never in a position to prove empirically that it is right.
Suppose, for example, we are trying to decide between (C) and (D). The advocates of (C) would round up all the noblest metals to test their definition. The advocates of (D) would round up all the elemental stuff with atomic number 79 to test theirs. Neither camp could ever persuade the other that their essential definition was correct, because, on the basis of their respective definitions, each would reject exactly those particular substances that the other took as decisive evidence in favor of his or her definition. As Heidegger summarizes the situation, “every time we attempt to prove an essential determination through single, or even all, actual and possible facts, there results the remarkable state of affairs that we have already presupposed the legitimacy of the essential determination, indeed must presuppose it, just in order to grasp and produce the facts that are supposed to serve as proof” (GA 45: 79).