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' in 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 the word which come into being at the same
time.