Objection 1. It would seem that man cannot attain happiness. For just as the rational is above the sensible nature, so the intellectual is above the rational, as Dionysius declares (Div. Nom. iv, vi, vii) in several passages. But irrational animals that have the sensitive nature only, cannot attain the end of the rational nature. Therefore neither can man, who is of rational nature, attain the end of the intellectual nature, which is Happiness.
Objection 2. Further, True Happiness consists in seeing God, Who is pure Truth. But from his very nature, man considers truth in material things: wherefore "he understands the intelligible species in the phantasm" (De Anima iii, 7). Therefore he cannot attain Happiness.
Objection 3. Further, Happiness consists in attaining the Sovereign Good. But we cannot arrive at the top without surmounting the middle. Since, therefore, the angelic nature through which man cannot mount is midway between God and human nature; it seems that he cannot attain Happiness.
On the contrary, It is written (Psalm 93:12): "Blessed is the man whom Thou shalt instruct, O Lord."
I answer that, Happiness is the attainment of the Perfect Good. Whoever, therefore, is capable of the Perfect Good can attain Happiness. Now, that man is capable of the Perfect Good, is proved both because his intellect can apprehend the universal and perfect good, and because his will can desire it. And therefore man can attain Happiness. This can be proved again from the fact that man is capable of seeing God, as stated in I, 12, 1: in which vision, as we stated above (Question 3, Article 8) man's perfect Happiness consists.
Reply to Objection 1. The rational exceeds the sensitive nature, otherwise than the intellectual surpasses the rational. For the rational exceeds the sensitive nature in respect of the object of its knowledge: since the senses have no knowledge whatever of the universal, whereas the reason has knowledge thereof. But the intellectual surpasses the rational nature, as to the mode of knowing the same intelligible truth: for the intellectual nature grasps forthwith the truth which the rational nature reaches by the inquiry of reason, as was made clear in the I, 58, 3; I, 79, 8. Therefore reason arrives by a kind of movement at that which the intellect grasps. Consequently the rational nature can attain Happiness, which is the perfection of the intellectual nature: but otherwise than the angels. Because the angels attained it forthwith after the beginning of their creation: whereas man attains if after a time. But the sensitive nature can nowise attain this end.
Reply to Objection 2. To man in the present state of life the natural way of knowing intelligible truth is by means of phantasms. But after this state of life, he has another natural way, as was stated in the I, 84, 7; I, 89, 1.
Reply to Objection 3. Man cannot surmount the angels in the degree of nature so as to be above them naturally. But he can surmount them by an operation of the intellect, by understanding that there is above the angels something that makes men happy; and when he has attained it, he will be perfectly happy.
Source: Summa Theologica (New Advent)