From _Opposita_ Aristotle passes to _Prius_ and _Simul_; with the different modes of each.[145] _Successive_ and _Synchronous_, are the two most general classes under which facts or events can be cast. They include between them all that is meant by Order in Time. They admit of no definition, and can be explained only by appeal to immediate consciousness in particular cases. Priority and Simultaneity, in this direct and primary sense, are among the clearest and most impressive notions of the human mind. But Aristotle recognizes four additional meanings of these same words, which he distinguishes from the primary, in the same way as he distinguishes (in the ten Categories) the different meanings of _Essentia_, in a gradually descending scale of analogy. The secondary _Prius_ is that which does not reciprocate according to the order of existence with its _Posterius_; where the _Posterius_ presupposes the _Prius_, while the _Prius_ does not presuppose the _Posterius_: for example, given two, the existence of one is necessarily implied; but given one, the existence of two is not implied.[146] The tertiary _Prius_ is that which comes first in the arrangements of science or discourse: as, in geometry, point and line are prior as compared with the diagrams and demonstrations; in writing, letters are prior as compared with syllables; in speeches, the proem is prior as compared with the exposition. A fourth mode of _Prius_ (which is the most remote and far-fetched) is, that the better and more honourable is _prius naturâ_. Still a fifth mode is, when, of two Relatives which reciprocate with each other as to existence, one is cause and the other effect: in such a case, the cause is said to be prior by nature to the effect.[147] For example, if it be a fact that Caius exists, the proposition "Caius exists," is a true proposition; and _vice versâ_, if the proposition "Caius exists" is a true proposition, it is a fact that Caius exists. But though from either **of these you can infer the other, the truth of the proposition is the effect, and not the cause, of the reality of the fact. Hence it is correct to say that the latter is _prius naturâ_, and the former _posterius naturâ_. [Footnote 145: Categ. p. 14, a. 26, seq.] [Footnote 146: Ibid. p. 14, a. 29, seq. This second mode of _Prius_ is entitled by Alexander (see Schol. (ad Metaphys. [Greek: D].) p. 707, b. 7, Brandis) [Greek: pro/teron tê=| phu/sei]. But Aristotle does not so call it here; he reserves that title for the fourth and fifth modes. It appears that debates, [Greek: Peri\ Prote/rou kai\ U(ste/rou] were frequent in the dialectic schools of Aristotle's day as well as debates, [Greek: Peri\ Tau)tou= kai\ E(te/rou, Peri\ O(moi/ou kai\ A)nomoi/ou, Peri\ Tau)to/têtos kai\ E)nantio/têtos] (Arist. Metaph. B. p. 995, b. 20).] [Footnote 147: Aristot. Categ. p. 14, b. 10.]