2. _Natural Philosophy._ In his system Heraclitus did not rest content with thus expressing himself in Notions, or with what is purely logical. But in addition to this universal form in which he advanced his principle, he gave his idea a real and more natural form, and hence he is still reckoned as belonging to the Ionic school of natural philosophers. However, as regards this form of reality, historians are at variance; most of them, and amongst others, Aristotle (Met. I. 3, 8), say that he maintained fire to be the existent principle; others, according to Sextus (adv. Math. IX. 360; X. 233), say it was air, and others again assert that he made vapour to be the principle rather than air;[62] even time is, in Sextus (adv. Math. X. 216), given as the primary existence. The question arises as to how this diversity is to be comprehended. It must not be believed that all these accounts are to be ascribed to the inaccuracy of historians, for the witnesses are of the best, like Aristotle and Sextus Empiricus, who do not speak casually of these forms, but definitely, without, however, remarking upon any such differences and contradictions. We seem to have a better reason in the obscurity of the writing of Heraclitus, which might, by the confusion of its expression, give occasion to misunderstanding. But when regarded closer, this difficulty, which is evident when merely looked at superficially, disappears; it is in the profoundly significant conceptions of Heraclitus that the true way out of this difficulty manifests itself. Heraclitus could no longer, like Thales, express water, air or anything similar as an absolute principle—he could no longer do so in the form of a primeval element from which the rest proceeds—because he thought of Being as identical with non-being, or the infinite Notion; thus the existent, absolute principle cannot with him come forth as a definite and actual thing such as water, but must be water in alteration, or as process only.