In the case of contraries, it is not always necessary that if one exists the other should also exist: for if all become healthy there will be health and no disease, and again, if everything turns white, there will be white, but no black. Again, since the fact that Socrates is ill is the contrary of the fact that Socrates is well, and two contrary conditions cannot both obtain in one and the same individual at the same time, both these contraries could not exist at once: for if that Socrates was well was a fact, then that Socrates was ill could not possibly be one.
It is plain that contrary attributes must needs be present in subjects which belong to the same species or genus. Disease and health require as their subject the body of an animal; white and black require a body, without further qualification; justice and injustice require as their subject the human soul.
Moreover, it is necessary that pairs of contraries should in all cases either belong to the same genus or belong to contrary generaor be themselves genera. White and black belong to the same genus,colour; justice and injustice, to contrary genera, virtue and vice; while good and evil do not belong to genera, but are themselves actual genera, with terms under them.
There are four senses in which one thing can be said to be "prior" to another. Primarily and most properly the term has reference to time: in this sense the word is used to indicate that one thing is older or more ancient than another, for the expressions "older" and "more ancient" imply greater length of time.
Secondly, one thing is said to be "prior" to another when the sequence of their being cannot be reversed. In this sense "one" is "prior" to "two". For if "two" exists, it follows directly that "one" must exist, but if "one" exists, it does not follow necessarily that "two" exists: thus the sequence subsisting cannot be reversed. It is agreed, then, that when the sequence of two things cannot be reversed, then that one on which the other depends is called "prior" to that other.
In the third place, the term "prior" is used with reference to anyorder, as in the case of science and of oratory. For in sciences whichuse demonstration there is that which is prior and that which isposterior in order; in geometry, the elements are prior to the propositions; in reading and writing, the letters of the alphabet are prior to the syllables. Similarly, in the case of speeches, the exordium is prior in order to the narrative.
Besides these senses of the word, there is a fourth. That which is better and more honourable is said to have a natural priority. In common parlance men speak of those whom they honour and love as "coming first" with them. This sense of the word is perhaps the most far-fetched.
Such, then, are the different senses in which the term "prior" is used.
Yet it would seem that besides those mentioned there is yet another. For in those things, the being of each of which implies that of the other, that which is in any way the cause may reasonably be said to be by nature "prior" to the effect. It is plain that there are instances of this. The fact of the being of a man carries with it the truth of the proposition that he is, and the implication is reciprocal: for if a man is, the proposition wherein we allege that he is true, and conversely, if the proposition wherein we allege that he is true, then he is. The true proposition, however, is in no way the cause of the being of the man, but the fact of the man"s being does seem somehow to be the cause of the truth of the proposition, forthe truth or falsity of the proposition depends on the fact of the man"s being or not being.
Thus the word "prior" may be used in five senses.
The term "simultaneous" is primarily and most appropriately applied to those things the genesis of the one of which is simultaneous with that of the other; for in such cases neither is prior or posterior to the other. Such things are said to be simultaneous in point of time. Those things, again, are "simultaneous" in point of nature, the being of each of which involves that of the other, while at the same time neither is the cause of the other"s being. This is the case with regard to the double and the half, for these are reciprocally dependent, since, if there is a double, there is also a half, and if there is a half, there is also a double, while at the same time neither is the cause of the being of the other.
Again, those species which are distinguished one from another and opposed one to another within the same genus are said to be "simultaneous" in nature. I mean those species which are distinguished each from each by one and the same method of division.