The first question is: πότερον τῶν ὄντων ἐστίν ἢ τῶν μὴ ὄντων;4 does time belong among beings or non-beings? Is it something that exists of itself or does it exist only in such a way that it is co-present in something that exists independently? How and where is time? The second question runs: τίς ἡ φύσις αὐτοῦ;5 what is the nature, the essence, of time? These two questions about time's mode of being and its essential nature receive proportionately unequal treatment. The first question is discussed in lesser detail; the positive answer is given only in the last chapter (14.223a 16-224a 17). The remaining portions of the treatise are devoted to the investigation and discussion of the second question, What is time? Chapter 10 not only defines both these problems but also discusses provisionally the difficulties implicit in them, and in connection with this it makes reference to previous attempts at a solution. Aristotle's custom is almost without exception to introduce his investigations in this form: historical orientation and discussion of the difficulties, the aporiai. Ἀπορία means: not getting through, being without passage. The problems are at first set in such a way that it appears as though no further passage can be made in these inquiries. The essential content of the problem is provisionally brought closer by this historical orientation and discussion of aporiai.
With reference to the first question, whether time is something extant or is not rather a me on, the latter determination seems to suggest itself as the answer. How should time exist as a whole, an οὐσία, if the parts that go to make it up are non-existent and are so in different ways? Things past and things future belong to time. The former are no longer, the latter are not yet. Past and future have the character of a nullity. It is as though time, as Lotze once put it, has two arms which it stretches out in different directions of non-being. Past and future, by their very concepts, are exactly non-existent; at bottom it is only the present, the now, that is. But on the other hand, time also is not composed of a manifold of existent nows. For in every now there is only this now, and the others are now either not yet or no longer. The now also is never the same and never a single one, but another, a not-the-same and not-one, a manifold. But selfsameness and unity are determinations necessarily belonging to something that exists in itself. If these determinations themselves are lacking to perhaps the only moment of time of which it
4. Aristotle, Physica (Ross), book 4, 10.217b 31. [W. D. Ross's edition, or editions, of Aristotle's Physics can be traced back, at the earliest, to 1936: Aristotle's Physics, with introduction and commentary (Oxford: Clarendon, 1936). Heidegger could not have used this text, and the data provided in the Grundprobleme text (note 4, page 330) are insufficient to determine which edition is intended. Heidegger could have used the editions by Immanuel Bekker (Berlin, 1843) or Charles Prantl (Leipzig: Teubner, 1879). See note 34 below.]
5. Ibid., 217b 32.