The Complete Aristotle (eng.)

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
any order, as in the case of science and of oratory. For in
sciences which use demonstration there is that which is prior and
that which is posterior 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, for the 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.
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    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. Thus the ‘winged’ species is simultaneous with the
‘terrestrial’ and the ‘water’ species. These are distinguished
within the same genus, and are opposed each to each, for the genus
‘animal’ has the ‘winged’, the ‘terrestrial’, and the ‘water’
species, and no one of these is prior or posterior to another; on
the contrary, all such things appear to be ‘simultaneous’ in
nature. Each of these also, the terrestrial, the winged, and the
water species, can be divided again into subspecies. Those species,
then, also will be ‘simultaneous’ point of nature, which, belonging
to the same genus, are distinguished each from each by one and the
same method of differentiation.
    But genera are prior to species, for the sequence of their being
cannot be reversed. If there is the species ‘water-animal’, there
will be the genus ‘animal’, but granted the being of the genus
‘animal’, it does not follow necessarily that there will be the
species ‘water-animal’.
    Those things, therefore, are said to be ‘simultaneous’ in
nature, the being of each of which involves that of the other,
while at the same time neither is in any way the cause of the
other’s being; those species, also, which are distinguished each
from each and opposed within the same genus. Those things,
moreover, are ‘simultaneous’ in the unqualified sense of