Objection 1. It would seem that it is proper to the rational nature to act for an end. For man, to whom it belongs to act for an end, never acts for an unknown end. On the other hand, there are many things that have no knowledge of an end; either because they are altogether without knowledge, as insensible creatures: or because they do not apprehend the idea of an end as such, as irrational animals. Therefore it seems proper to the rational nature to act for an end.
Objection 2. Further, to act for an end is to order one's action to an end. But this is the work of reason. Therefore it does not belong to things that lack reason.
Objection 3. Further, the good and the end is the object of the will. But "the will is in the reason" (De Anima iii, 9). Therefore to act for an end belongs to none but a rational nature.
On the contrary, The Philosopher proves (Phys. ii, 5) that "not only mind but also nature acts for an end."
I answer that, Every agent, of necessity, acts for an end. For if, in a number of causes ordained to one another, the first be removed, the others must, of necessity, be removed also. Now the first of all causes is the final cause. The reason of which is that matter does not receive form, save in so far as it is moved by an agent; for 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, 22, 2, ad 4; I, 103, 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 to Objection 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 to Objection 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 to Objection 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.
Source: Summa Theologica (New Advent)