nothing reduces itself from potentiality to act. But an agent does
not move except out of intention for an end. For if the agent were
not determinate to some particular effect, it would not do one thing
rather than another: consequently in order that it produce a
determinate effect, it must, of necessity, be determined to some
certain one, which has the nature of an end. And just as this
determination is effected, in the rational nature, by the "rational
appetite," which is called the will; so, in other things, it is
caused by their natural inclination, which is called the "natural
appetite."

Nevertheless it must be observed that a thing tends to an end, by its
action or movement, in two ways: first, as a thing, moving itself to
the end, as man; secondly, as a thing moved by another to the end, as
an arrow tends to a determinate end through being moved by the archer
who directs his action to the end. Therefore those things that are
possessed of reason, move themselves to an end; because they have
dominion over their actions through their free-will, which is the
"faculty of will and reason." But those things that lack reason tend
to an end, by natural inclination, as being moved by another and not
by themselves; since they do not know the nature of an end as such,
and consequently cannot ordain anything to an end, but can be
ordained to an end only by another. For the entire irrational nature
is in comparison to God as an instrument to the principal agent, as
stated above (I, Q. 22, A. 2, ad 4; Q. 103, A. 1, ad 3). Consequently
it is proper to the rational nature to tend to an end, as directing
(_agens_) and leading itself to the end: whereas it is proper to the
irrational nature to tend to an end, as directed or led by another,
whether it apprehend the end, as do irrational animals, or do not
apprehend it, as is the case of those things which are altogether