Objection 1. It would seem that man's happiness consists in the knowledge of separate substances, namely, angels. For Gregory says in a homily (xxvi in Evang.): "It avails nothing to take part in the feasts of men, if we fail to take part in the feasts of angels"; by which he means final happiness. But we can take part in the feasts of the angels by contemplating them. Therefore it seems that man's final happiness consists in contemplating the angels.
Objection 2. Further, the final perfection of each thing is for it to be united to its principle: wherefore a circle is said to be a perfect figure, because its beginning and end coincide. But the beginning of human knowledge is from the angels, by whom men are enlightened, as Dionysius says (Coel. Hier. iv). Therefore the perfection of the human intellect consists in contemplating the angels.
Objection 3. Further, each nature is perfect, when united to a higher nature; just as the final perfection of a body is to be united to the spiritual nature. But above the human intellect, in the natural order, are the angels. Therefore the final perfection of the human intellect is to be united to the angels by contemplation.
On the contrary, It is written (Jeremiah 9:24): "Let him that glorieth, glory in this, that he understandeth and knoweth Me." Therefore man's final glory or happiness consists only in the knowledge of God.
I answer that, As stated above (Article 6), man's perfect happiness consists not in that which perfects the intellect by some participation, but in that which is so by its essence. Now it is evident that whatever is the perfection of a power is so in so far as the proper formal object of that power belongs to it. Now the proper object of the intellect is the true. Therefore the contemplation of whatever has participated truth, does not perfect the intellect with its final perfection. Since, therefore, the order of things is the same in being and in truth (Metaph ii, 1); whatever are beings by participation, are true by participation. Now angels have being by participation: because in God alone is His Being His Essence, as shown in the I, 44, 1. It follows that contemplation of Him makes man perfectly happy. However, there is no reason why we should not admit a certain imperfect happiness in the contemplation of the angels; and higher indeed than in the consideration of speculative science.
Reply to Objection 1. We shall take part in the feasts of the angels, by contemplating not only the angels, but, together with them, also God Himself.
Reply to Objection 2. According to those that hold human souls to be created by the angels, it seems fitting enough, that man's happiness should consist in the contemplation of the angels, in the union, as it were, of man with his beginning. But this is erroneous, as stated in I, 90, 3. Wherefore the final perfection of the human intellect is by union with God, Who is the first principle both of the creation of the soul and of its enlightenment. Whereas the angel enlightens as a minister, as stated in the I, 111, 2, ad 2. Consequently, by his ministration he helps man to attain to happiness; but he is not the object of man's happiness.
Reply to Objection 3. The lower nature may reach the higher in two ways. First, according to a degree of the participating power: and thus man's final perfection will consist in his attaining to a contemplation such as that of the angels. Secondly, as the object is attained by the power: and thus the final perfection of each power is to attain that in which is found the fulness of its formal object.
Source: Summa Theologica (New Advent)