Therefore knowledge does not need to be constantly carried out, I do not have to look constantly at the known beings. On the contrary, the knowing is a tarrying being-present to beings, a disposition toward their uncoveredness, even if I do not stand before them. My knowledge is secure because these beings always are. I do not have to return to them again and again. Hence I have no knowledge of beings which can be otherwise-and that is the reason for saying that what is historical cannot be known in a proper sense. This mode of the ἀληθεύειν of ἐπιστήμη is a wholly determinate one, for the Greeks surely the one which grounds the possibility of science. The entire further development of science and today's theory of science take their orientation from this concept of knowledge.
This concept is not deduced but is intuited on the basis of the full phenomenon of knowing. Precisely there we find that knowing is a preserving of the uncoveredness of beings, ones which are independent of it and yet are at its disposal. The knowable, however, which I have at my disposal, must necessarily be as it is; it must always be so; it is the being that always is so, that which did not become, that which never was not and never will not be; it is constantly so; it is a being in the most proper sense.
Now that is remarkable: beings are determined with regard to their Being by a moment of time. The everlasting characterizes beings with regard to their Being. The ὄντα are ἀίδια (b23f.). Ἀίδιον belongs to the same stem as ἀεί and αἰών. καὶ γὰρ τὸ ἀεὶ συνεχές (Phys. Θ, 6, 259al6f.). Ἀεί, "always, everlasting," is "that which coheres in itself, that which is never interrupted." Αἰών means the same as lifetime, understood as full presence: τὸν ἅπαντα αιώνα (De Caelo A, 9, 279b22). Every living being has its αἰών, its determinate time of presence. Αἰών expresses the full measure of presence, of which a living being disposes. In a broader sense, αἰών signifies the duration of the world in general, and indeed according to Aristotle the world is eternal; it did not come into being and is imperishable. The existence of what is alive as well as of the world as a whole is hence determined as αἰών. And the οὐρανός determines for the living thing its αἰών, its presence. Furthermore, the ἀΐδια are πρότερα τῇ οὐσίᾳ τῶν φθαρτῶν (Met. Θ, 8, 1050b7): "what always is is, with regard to presence, earlier than what is perishable," earlier than what once came into being and hence was once not present. Therefore καὶ ἐξ ἀρχῆς καὶ τὰ ἀΐδια (cf. 1051a19f.), the ἀΐδια are what form the beginning for all other beings. They are therefore that which properly is. For what the Greeks mean by Being is presence, being in the present. Therefore that which always dwells in the now is most properly a being and is the ἀρχή, the origin, of the rest of beings. All determinations of beings can be led back, if necessary, to an everlasting being and are intelligible on that basis.