On the other hand, Aristotle stresses: τὰ ἀεὶ ὄντα, ᾗ ἀεὶ ὄντα, οὐκ ἔστιν ἐν χρόνῳ (Phys. Δ, 12, 221b3ff.). "That which always is, insofar as it always is, is not in time." οὐδὲ μετρεῖται τὸ εἶναι αὐτῶν ὑπὸ τοῦ χρόνου (ibid.), "it suffers nothing from time," it is unchangeable. And yet Aristotle also maintains that precisely the heavens are eternal, αἰών, and specifically eternal in the sense of sempiternitas, not in the sense of aeternitas. Here in Physics Δ, 12, on the contrary, he says that the ἀεὶ ὄντα are not in time. Nevertheless, Aristotle provides a precise clarification of what he understands by "in time." To be in time means τὸ μετρεῖσθαι τὸ εἶναι ὑπὸ τοῦ χρόνου (cf. b5), "to be measured by time with regard to Being." Aristotle hence does not have some sort of arbitrary and average concept of "in time." Instead, everything measured by time is in time. But something is measured by time insofar as its nows are determined: now and now in succession. But as to what always is, what is constantly in the now—its nows are numberless, limitless, ἄπειρον. Because the infinite nows of the ἀίδιον are not measurable, the ἀίδιον, the eternal, is not in time. But that does not make it "supertemporal" in our sense. What is not in time is for Aristotle still temporal, i.e., it is determined on the basis of time—just as the ἀίδιον, which is not in time, is determined by the ἄπειρον of the nows.
We have to hold fast to what is distinctive here, namely, that beings are interpreted as to their Being on the basis of time. The beings of ἐπιστήμη are the ἀεὶ ὄν. This is the first determination of the ἐπιστητόν.
b) The position of the ἀρχή in ἐπιστήμη (Nic. Eth. VI, 3;
Post. An. I, 1). The teachability of ἐπιστήμη. Ἀπόδειξις and
ἐπαγωγή. The presupposition of the ἀρχή.
The second determination of the ἐπιστητόν is found first in the Nicomachean Ethics VI, 6: the ἐπιστητόν is ἀποδεικτόν (1140b35). Here (VI, 3) that is expressed as follows: ἐπιστήμη is διδακτή (139b25-35), "teachable"; the ἐπιστητόν, the knowable as such, is μαθητόν (b25f.), learnable. It pertains to knowledge that one can teach it, i.e., impart it, communicate it. This is a constitutive determination of knowledge, and not only of knowledge but of τέχνη as well.1 In particular, scientific knowledge is ἐπιστήμη μαθηματική. And the μαθηματικαὶ τῶν ἐπιστημῶν (71a3), mathematics, is teachable in a quite preeminent sense. This teachability makes clear what is involved in knowledge. Knowledge is a positionality toward beings which has their uncoveredness available without being constantly present to them. Knowledge is teachable, i.e., it is communicable, without there having to take place an uncovering in the propper sense.
1. Cf., on the following, Post. An. I, 1, 71a2ff.