Objection 1. It would seem that man chooses of necessity. For the end stands in relation to the object of choice, as the principle of that which follows from the principles, as declared in Ethic. vii, 8. But conclusions follow of necessity from their principles. Therefore man is moved of necessity from (willing) the end of the choice (of the means).
Objection 2. Further, as stated above (01, ad 2), choice follows the reason's judgment of what is to be done. But reason judges of necessity about some things: on account of the necessity of the premises. Therefore it seems that choice also follows of necessity.
Objection 3. Further, if two things are absolutely equal, man is not moved to one more than to the other; thus if a hungry man, as Plato says (Cf. De Coelo ii, 13), be confronted on either side with two portions of food equally appetizing and at an equal distance, he is not moved towards one more than to the other; and he finds the reason of this in the immobility of the earth in the middle of the world. Now, if that which is equally (eligible) with something else cannot be chosen, much less can that be chosen which appears as less (eligible). Therefore if two or more things are available, of which one appears to be more (eligible), it is impossible to choose any of the others. Therefore that which appears to hold the first place is chosen of necessity. But every act of choosing is in regard to something that seems in some way better. Therefore every choice is made necessarily.
On the contrary, Choice is an act of a rational power; which according to the Philosopher (Metaph. ix, 2) stands in relation to opposites.
I answer that, Man does not choose of necessity. And this is because that which is possible not to be, is not of necessity. Now the reason why it is possible not to choose, or to choose, may be gathered from a twofold power in man. For man can will and not will, act and not act; again, he can will this or that, and do this or that. The reason of this is seated in the very power of the reason. For the will can tend to whatever the reason can apprehend as good. Now the reason can apprehend as good, not only this, viz. "to will" or "to act," but also this, viz. "not to will" or "not to act." Again, in all particular goods, the reason can consider an aspect of some good, and the lack of some good, which has the aspect of evil: and in this respect, it can apprehend any single one of such goods as to be chosen or to be avoided. The perfect good alone, which is Happiness, cannot be apprehended by the reason as an evil, or as lacking in any way. Consequently man wills Happiness of necessity, nor can he will not to be happy, or to be unhappy. Now since choice is not of the end, but of the means, as stated above (Article 3); it is not of the perfect good, which is Happiness, but of other particular goods. Therefore man chooses not of necessity, but freely.
Reply to Objection 1. The conclusion does not always of necessity follow from the principles, but only when the principles cannot be true if the conclusion is not true. In like manner, the end does not always necessitate in man the choosing of the means, because the means are not always such that the end cannot be gained without them; or, if they be such, they are not always considered in that light.
Reply to Objection 2. The reason's decision or judgment of what is to be done is about things that are contingent and possible to us. In such matters the conclusions do not follow of necessity from principles that are absolutely necessary, but from such as are so conditionally; as, for instance, "If he runs, he is in motion."
Reply to Objection 3. If two things be proposed as equal under one aspect, nothing hinders us from considering in one of them some particular point of superiority, so that the will has a bent towards that one rather than towards the other.
Source: Summa Theologica (New Advent)