It is clear to start with, that in virtue of his very rationality man is able to use other things as his instruments. They may destroy him eventually by sheer brute force, but they can never use him; on the contrary, he uses them. Things, therefore, are ordained to man as to their end, not man to things, and that amounts to saying that the rest of the universe is directed towards its end by man and through man. Reasonable beings are there, in a sense, for their own sake, the rest are there only for the sake of reasonable beings. The case is like that of an army, of which the whole object is the achievement of victory; those who are to achieve it are the soldiers who fight, and as for the auxiliary services, they exist only for the soldiers and participate in the victory only through the soldiers. it is the same with the universe and man; for the end of the universe is beatitude, and since only reasonable beings can enjoy it, the rest are called to participate it in them and for their sake. Providence, therefore has specially chosen the human species and leads it towards its end in an altogether special manner, since God is the end of the universe and it is through humanity that it is to attain Him. (Ch. 8)
Gilson points to the utility men make of other things as the key empirical fact separating man from the rest of material creation. Man uses things, but they never use him, although they may destroy him. This undeniable phenomenon cuts through the ongoing debates respecting the metaphysical status of animals, e.g. to what extent they share rationality with us. Whatever extent it is, it is never to the extent that animals domesticate men the way men domesticate animals. And the relationship between men and animals is one of instrumental to final ends; man uses other things for his own ends. This is not a metaphysical conclusion but a simple observation of empirical fact; but from that fact significant conclusions, including the metaphysical, may be drawn. The relationship between men and animals is itself constituted by nature - man did not set himself up as lord of the earth but it is nature that makes him so. Since this relationship is constituted by nature it reflects nature's ends as well as man's. That man is an end for nature in a way superior to other things is not merely an anthropological conceit. Nature proclaims it so in its very constitution.
And note that it is nature that does so. Gilson does not quote Scripture. This makes it a "humanism" not only because it ratifies the essential dignity of man, but because it does so on the basis of man's own nature as we know it independently of revelation. Yet it is a Christian humanism because, although based on an empirical analysis of man's nature, it provides a ground in which the specifically Christian understanding of Providence makes sense. God has specially provided for man not to separate man from nature but because man is nature's glory; and it is through man that nature attains to God.