on of the builder. The same principle
applies to natural things. For if an agent is not the cause of a form
as such, neither will it be directly the cause of _being_ which
results from that form; but it will be the cause of the effect, in
its _becoming_ only.

Now it is clear that of two things in the same species one cannot
directly cause the other's form as such, since it would then be the
cause of its own form, which is essentially the same as the form of
the other; but it can be the cause of this form for as much as it is
in matter--in other words, it may be the cause that "this matter"
receives _this form._ And this is to be the cause of _becoming,_ as
when man begets man, and fire causes fire. Thus whenever a natural
effect is such that it has an aptitude to receive from its active
cause an impression specifically the same as in that active cause,
then the _becoming_ of the effect, but not its _being,_ depends on
the agent.

Sometimes, however, the effect has not this aptitude to receive the
impression of its cause, in the same way as it exists in the agent:
as may be seen clearly in all agents which do not produce an effect
of the same species as themselves: thus the heavenly bodies cause the
generation of inferior bodies which differ from them in species. Such
an agent can be the cause of a form as such, and not merely as
existing in this matter, consequently it is not merely the cause of
_becoming_ but also the cause of _being._

Therefore as the becoming of a thing cannot continue when that action
of the agent ceases which causes the _becoming_ of the effect: so
neither can the _being_ of a thing continue after that action of the
agent has ceased, which is the cause of the effect not only in
_becoming_ but also in _being._ This is why hot water retains heat
after the cessation of the fire's action; while, on the contrary, the
air does not continue to be lit up, even for a moment, when the sun
ceases to act upon it, because water is a matter susceptive of the
fire's heat in the same way as it exists in the fire. Wherefore if it
were to be reduced to the perfect form of fire, it would retain that
form always; whereas if it has the form of fire imperfectly and
inchoately, the heat will remain for a time only, by reason of the
imperfect participation of the principle of heat. On the other hand,
air is not of such a nature as to receive light in the same way as it
exists in the sun, which is the principle of li