God is the end of every intellectual substance
Summa Contra Gentiles, Book III, c. 25
Since all creatures, even those devoid of understanding, are ordered to God as to an ultimate end, all achieve this end to the extent that they participate somewhat in His likeness. Intellectual creatures attain it in a more special way, that is, through their proper operation of understanding Him. Hence, this must be the end of the intellectual creature, namely, to understand God.
The ultimate end of each thing is God, as we have shown [See above, ch. 17] . So, each thing intends, as its ultimate end, to be united with God as closely as is possible for it. Now, a thing is more closely united with God by the fact that it attains to His very substance in some manner, and this is accomplished when one knows something of the divine substance, rather than when one acquires some likeness of Him. Therefore, an intellectual substance tends to divine knowledge as an ultimate end.
Again, the proper operation of a thing is an end for it, for this is its secondary perfection. That is why whatever is fittingly related to its proper operation is said to be virtuous and good. But the act of understanding is the proper operation of substance. Therefore, this act is its end. And that which is most perfect in this operation is the ultimate end, particularly in the case of operations that are not ordered to any products, such as the acts of understanding and sensing. Now, since operations of this type are specified any their objects, through which they are known also, any one of these operations must be more perfect when its object is more perfect. And so, to understand the most perfect intelligible object, which is God, is the most perfect thing in the genus of this operation of understanding. Therefore, to know God by an act of understanding is the ultimate end of every intellectual substance.
Of course, someone could say that the ultimate end of an intellectual substance consists, in fact, in understanding the best intelligible object - not that the best object of understanding for this or that particular intellectual substance is absolutely the best intelligible object, but that, the higher an intellectual substance is, the higher will its best object of understanding be. And so, perhaps the highest created intellectual substance may have what is absolutely best as its best intelligible object, and, consequently, its felicity will consist in understanding God, but the felicity of any lower intellectual substance will lie in the understanding of some lower intelligible object, which is, however, the highest thing understood by it. Particularly would it seem true of the human intellect that its function is not to understand absolutely the best intelligible object, because of its weakness; indeed, it stands in relation to the knowing of the greatest intelligible object, "as the owl's eye is to the sunlight" [Aristotle, Metaphysics, Iα, 1 (993b 9)].
But it seems obvious that the end of any intellectual substance, even the lowest, is to understand God. It has been shown above [See above, ch. 17] that the ultimate end of all things, to which they tend, is God. Though it is the lowest in the order of intellectual substances, the human intellect is, nevertheless, superior to all things that lack understanding. And so, since there should not be a less noble end for a more noble substance, the end for the human intellect will be God Himself. And an intelligent being attains his ultimate end by understanding Him, as was indicated. Therefore, the human intellect reaches God as its end, through an act of understanding.
Again, just as things devoid of understanding tend toward God as an end, by way of assimilation, so intellectual substances do so by way of cognition, as is evident from the foregoing. Now, although things devoid of understanding tend to the likeness of their proximate agents, their natural tendency does not, however, rest there, for this tendency has as its end assimilation to the highest good, as is apparent from what we have said [See above, ch. 19], even though these things can only attain this likeness in a very imperfect way. Therefore, however small the amount of divine knowledge that the inte1lect may be able to grasp, that will be for the intellect, in regard to its ultimate end, much more than the perfect knowledge of lower objects of understanding.
Besides, a thing has the greatest desire for its ultimate end. Now, the human intellect has a greater desire, and love, and pleasure, in knowing divine matters than it has in the perfect knowledge of the lowest things, even though it can grasp but little concerning divine things. So, the ultimate end of man is to understand God, in some fashion.
Moreover, a thing inclines toward the divine likeness as to its own end. So, that whereby a thing chiefly becomes like God is its ultimate end. Now, an intellectual creature chiefly becomes like God by the fact that it is intellectual, for it has this sort of likeness over and above what other creatures have, and this likeness includes all others. In the genus of this sort of likeness a being becomes more like God by actually understanding than by habitually or potentially understanding, because God is always actually understanding, as we proved in Book One [SCG, I, ch. 56]. And, in this actual understanding, it becomes most like God by understanding God Himself, for God understands all things in the act of understanding Himself, as we proved in Book One [SCG, I, ch. 49]. Therefore, to understand God is the ultimate end of every intellectual substance.
Furthermore, that which is capable of being loved only for the sake of some other objects exists for the shake of that other thing which is lovable simply on its own account. In fact, there is no point in going on without end in the working of natural appetite, since natural desire would then be futile, because it is impossible to get to the end of an endless series. Now, all practical sciences, arts, and powers are objects of love only because they are means to something else, for purpose is not knowledge but operation. But the speculative sciences are lovable for their own sake, since their end is knowledge itself. Nor do we find any action in human affairs, except speculative thought, that is not directed to some other end. Even sports activities, which appear to be carried on without any purpose, have a proper end, namely, so that after our minds have been somewhat relaxed through them we may be then better able to do serious jobs. Otherwise, if sport were an end in itself, the proper thing to do would be to play all the time, but that is not appropriate. So, the practical arts are ordered to the speculative ones, and likewise every human operation to intellectual speculation, as an end . Now, among all the sciences and arts which are thus subordinated, the ultimate end seems to belong to the one that is preceptive and architectonic in relation to the others. For instance, the art of navigation, to which the end, that is the use, of a ship pertains, is architectonic and preceptive in relation to the art of shipbuilding. In fact, this is the way that first philosophy is related to the other speculative sciences, for all the others depend on it, in the sense that they take their principles from it, and also the position to be assumed against those who deny the principles. And this first philosophy is wholly ordered to the knowing of God, as its ultimate end; that is why it is also called divine science [See Aristotle, Metaphysics, I, 2 (983a 6)]. So, divine knowledge is the ultimate end of every act of human knowledge and every operation.
Again, in all agents and movers that are arranged in an order, the end of the first agent and mover must be the ultimate end of all. Thus, the end of the commander of an army is the end of all who serve as soldiers under him. Now, of all the parts of man, the intellect is found to be the superior mover, for the intellect moves the appetite, by presenting it with its object; then the intellectual appetite, that is the will, moves the sensory appetites, irascible and concupiscible, and that is why we do not obey concupiscence unless there ne a command from the will; and finally, the sense appetite, with the advent of consent from the will, now moves the body. Therefore, the end of the intellect is the end of all human actions. "But the end and good of the intellect are the true" [See Aristotle, Nicomachean Ethics, VI, 2 (1139a 27)]; consequently, the first truth is the ultimate end . So, the ultimate end of the whole man, and of all his operations and desires, is to know the first truth, which is God.
Besides, there is naturally present in all men the disere to know the causes of whatever things are observed. Hence because of wondering about things that were seen but whose causes were hidden, men first began to think philosophically; when they found the cause, they were satisfied . But the search did not stop until it reached the first cause, for "then do we think that we know perfectly, when we know the first cause" [See Aristotle, Metaphysics, I, 3 (983a 25)]. Therefore, man naturally desires, as his ultimate end, to know the first cause. But the first cause of all things is God. Therefore, the ultimate end of man is to know God.
Moreover, for each effect that he knows, man naturally desires to know the cause. Now, the human intellect knows universal being. So, he naturally desires to know its cause, which is God alone, as we proved in Book Two [SCG, II, ch. 15]. Now, a person has not attained his ultimate end until natural desire comes to rest. Therefore, for human happiness which is the ultimate end it is not enough to have merely any kind of intelligible knowledge; there must be divine knowledge, as an ultimate end, to terminate the natural desire. So, the ultimate end of man is the knowledge of God.
Furthermore, a body tending toward its proper place by natural appetite is moved more forcibly and swiftly as it approaches its end. Thus, Aristotle proves, in On the Heavens I, [Aristotle, De caelo, I, 8 (277a 18)] that natural motion in a straight line cannot go on to infinity, for then it would be no more moved later than earlier. So, a thing that tends more forcibly later than earlier, toward an objective, is not moved toward an indefinite objective, but tends toward some determinate thing. Now, we find this situation in the desire to know. The more a person knows, the more is he moved by the desire to know. Hence, man's natural desire tends, in the process of knowing, toward some definite end. Now, this can be none other than the most noble object of knowledge, which is God. Therefore, divine knowledge is the ultimate end of man.
Now, the ultimate end of man, and of every intellectual substance, is called felicity or happiness, because this is what every intellectual substance desires as an ultimate end, and for its own sake alone. Therefore, the ultimate happiness and felicity of every intellectual substance is to know God.
And so, it is said in Matthew ( 5:8): "Blessed are the clean of heart, for they shall see God"; and in John (I 7:3): "This is eternal life, that they may know Thee, the only true God."
With this view, the judgment of Aristotle is also in agreement, in the last Book of his Ethics, where he says that the ultimate felicity of man is "speculative, in accord with the contemplation of the best object of speculation" [Aristotle, Nicomachean Ethics, X, 7 (1177a 18)].
Summa contra Gentiles, Book III, c. 25, translated by James F. Anderson (Notre Dame - London: University of Notre Dame Press, 1995), pp. 97-103.