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 void of knowledge.

Reply Obj. 1:  When a man of himself acts for an end, he knows the end:  but when he is directed or led by another, for instance, when he acts at another’s command, or when he is moved under another’s compulsion, it is not necessary that he should know the end.  And it is thus with irrational creatures.

Reply Obj. 2:  To ordain towards an end belongs to that which directs itself to an end:  whereas to be ordained to an end belongs to that which is directed by another to an end.  And this can belong to an irrational nature, but owing to some one possessed of reason.  Reply Obj. 3:  The object of the will is the end and the good in universal.  Consequently there can be no will in those things that lack reason and intellect, since they cannot apprehend the universal; but they have a natural appetite or a sensitive appetite, determinate to some particular good.  Now it is clear that particular causes are moved by a universal cause:  thus the governor of a city, who intends the common good, moves, by his command, all the particular departments of the city.  Consequently all things that lack reason are, of necessity, moved to their particular ends by some rational will which extends to the universal good, namely by the Divine will. ________________________

Third article [I-ii, Q. 1, Art. 3]

Whether Human Acts Are Specified by Their End?

Objection 1:  It would seem that human acts are not specified by their end.  For the end is an extrinsic cause.  But everything is specified by an intrinsic principle.  Therefore human acts are not specified by their end.

Obj. 2:  Further, that which gives a thing its species should exist before it.  But the end comes into existence afterwards.  Therefore a human act does not derive its species from the end.