On Generation and corruption
the 'elements', or else it is nothing.
Since, then, there is nothing-at least, nothing perceptible-prior to
these, they must be all. That being so, either they must always
persist and not be transformable into one another: or they must
undergo transformation-either all of them, or some only (as Plato
wrote in the Timacus).' Now it has been proved before that they must
undergo reciprocal transformation. It has also been proved that the
speed with which they come-to-be, one out of another, is not
uniform-since the process of reciprocal transformation is relatively
quick between the 'elements' with a 'complementary factor', but
relatively slow between those which possess no such factor.
Assuming, then, that the contrariety, in respect to which they are
transformed, is one, the elements' will inevitably be two: for it is
'matter' that is the 'mean' between the two contraries, and matter
is imperceptible and inseparable from them. Since, however, the
'elements' are seen to be more than two, the contrarieties must at the
least be two. But the contrarieties being two, the 'elements' must
be four (as they evidently are) and cannot be three: for the
couplings' are four, since, though six are possible, the two in
which the qualities are contrary to one another cannot occur.
These subjects have been discussed before:' but the following
arguments will make it clear that, since the 'elements' are
transformed into one another, it is impossible for any one of
them-whether it be at the end or in the middle-to be an 'originative
source' of the rest. There can be no such 'originative element' at the
ends: for all of them would then be Fire or Earth, and this theory
amounts to the assertion that all things are made of Fire or Earth.
Nor can a 'middle-element' be such an originative source'-as some
thinkers suppose that Air is transformed both into Fire and into
Water, and Water both into Air and into Earth, while the
'end-elements' are not further transformed into one another. For the
process must come to a stop, and cannot continue ad infinitum in a