Before we go look at that, though, we ought to consider briefly what he says about man as made in the image of God. In what does this image consist? I have heard others suggest that it consists in having dominion. Personally, I don't agree with this notion: exercising dominion is extrinsic to what we are, and it seems to me that we have examples of dominion after a fashion in the animal kingdom: witness so-called "alpha males" and "alpha females" among apes and wolves, for example, and territorialism among a variety of species. Clearly the quality of the dominion that these exercise is different from what a human might pursue, but it seems to me to be undeniable that we can identify animals which do seek to hold dominion after their fashion. But if the image of God in us is that which distinguishes us from the animals, then we ought to seek it in that which animals lack.
This is precisely what St. Thomas does. He says:
Augustine says (Gen. ad lit. vi, 12): "Man's excellence consists in the fact that God made him to His own image by giving him an intellectual soul, which raises him above the beasts of the field." Therefore things without intellect are not made to God's image.And because this is so important to what he will say in the second part of ST, he repeats it at the outset of I-II:
But some things are like to God first and most commonly because they exist; secondly, because they live; and thirdly because they know or understand; and these last, as Augustine says (QQ. 83, qu. 51) "approach so near to God in likeness, that among all creatures nothing comes nearer to Him." It is clear, therefore, that intellectual creatures alone, properly speaking, are made to God's image (ST I Q93 A2).
Since, as Damascene states (De Fide Orthod. ii. 12), man is said to be made to God's image, in so far as the image implies an intelligent being endowed with free-will and self-movement: now that we have treated of the exemplar, i.e., God, and of those things which came forth from the power of God in accordance with His will; it remains for us to treat of His image, i.e., man, inasmuch as he too is the principle of his actions, as having free-will and control of his actions (source).Having laid this foundation, we can go on to look at St. Thomas' understanding of what Human Acts are.
Of actions done by man those alone are properly called "human," which are proper to man as man. Now man differs from irrational animals in this, that he is master of his actions. Wherefore those actions alone are properly called human, of which man is master. Now man is master of his actions through his reason and will; whence, too, the free-will is defined as "the faculty and will of reason." Therefore those actions are properly called human which proceed from a deliberate will. And if any other actions are found in man, they can be called actions "of a man," but not properly "human" actions, since they are not proper to man as man. Now it is clear that whatever actions proceed from a power, are caused by that power in accordance with the nature of its object. But the object of the will is the end and the good. Therefore all human actions must be for an end (ST I-II Q1 A1).This is the best starting point that I have ever seen for a proper understanding of morality.