Having thus stated that _possible to be_, in this unilateral and equivocal sense but in no other, is a legitimate consequence of _necessary to be_, Aristotle proceeds to lay down a tripartite distinction which surprises us in this place. "It is plain from what has been said that that which is by Necessity, is in Act or Actuality; so that if things sempiternal are prior, Actuality is prior to Possibility. Some things, like the first (or celestial) substances, are Actualities without Possibility; others (the generated and perishable substances) which are prior in nature but posterior in generation, are Actualities along with Possibility; while a third class are Possibilities only, and never come into Actuality" (such as the largest number, or the least magnitude).[43] [Footnote 43: Ibid. p. 23, a. 21-26.] Now the sentence just translated (enunciating a doctrine of Aristotle's First Philosophy rather than of Logic) appears decidedly to contradict what he had said three lines before, viz., that in one certain sense, the _necessary to be_ included and implied the _possible to be_; that is, a possibility or potentiality unilateral only, not bilateral; for we are here told that the celestial substance is Actuality without Possibility (or Potentiality), so that the unilateral sense of this last term is disallowed. On the other hand, a third sense of the same term is recognized and distinguished; a sense neither bilateral nor unilateral, but the negation of both. This third sense is hardly intelligible, giving as it does an _impossible_ Possible; it seems a self-contradictory description.[44] At best, it can only be understood as a limit in the mathematical sense; a terminus towards which potentiality may come constantly nearer and nearer, but which it can never reach. The first, or bilateral potentiality, is the only sense at once consistent, legitimate, and conformable to ordinary speech. Aristotle himself admits that the second and third are equivocal meanings,[45] departing from the first as the legitimate meaning; but if equivocal departure to so great an extent were allowed, the term, put to such multifarious service, becomes unfit for accurate philosophical reasoning. And we find this illustrated by the contradiction into which Aristotle himself falls in the course of a few lines. The sentence of First Philosophy (which I translated in the last page) is a correction of the logical statement immediately preceding it, in so far as it suppresses the _necessary_ Possible, or the unilateral potentiality. But on the other hand the same sentence introduces a new confusion by its third variety--the _impossible_ Potential, departing from all clear and consistent meaning of potentiality, and coinciding only with the explanation of _Non-Ens_, as given by Aristotle elsewhere.[46]