Objection 1. It would seem that man's happiness consists in the consideration of speculative sciences. For the Philosopher says (Ethic. i, 13) that "happiness is an operation according to perfect virtue." And in distinguishing the virtues, he gives no more than three speculative virtues--"knowledge," "wisdom" and "understanding," which all belong to the consideration of speculative sciences. Therefore man's final happiness consists in the consideration of speculative sciences.
Objection 2. Further, that which all desire for its own sake, seems to be man's final happiness. Now such is the consideration of speculative sciences; because, as stated in Metaph. i, 1, "all men naturally desire to know"; and, a little farther on (2), it is stated that speculative sciences are sought for their own sakes. Therefore happiness consists in the consideration of speculative sciences.
Objection 3. Further, happiness is man's final perfection. Now everything is perfected, according as it is reduced from potentiality to act. But the human intellect is reduced to act by the consideration of speculative sciences. Therefore it seems that in the consideration of these sciences, man's final happiness consists.
On the contrary, It is written (Jeremiah 9:23): "Let not the wise man glory in his wisdom": and this is said in reference to speculative sciences. Therefore man's final happiness does not consist in the consideration of these.
I answer that, As stated above (02, ad 4), man's happiness is twofold, one perfect, the other imperfect. And by perfect happiness we are to understand that which attains to the true notion of happiness; and by imperfect happiness that which does not attain thereto, but partakes of some particular likeness of happiness. Thus perfect prudence is in man, with whom is the idea of things to be done; while imperfect prudence is in certain irrational animals, who are possessed of certain particular instincts in respect of works similar to works of prudence.
Accordingly perfect happiness cannot consist essentially in the consideration of speculative sciences. To prove this, we must observe that the consideration of a speculative science does not extend beyond the scope of the principles of that science: since the entire science is virtually contained in its principles. Now the first principles of speculative sciences are received through the senses, as the Philosopher clearly states at the beginning of the Metaphysics (i, 1), and at the end of the Posterior Analytics (ii, 15). Wherefore the entire consideration of speculative sciences cannot extend farther than knowledge of sensibles can lead. Now man's final happiness, which is his final perfection cannot consist in the knowledge of sensibles. For a thing is not perfected by something lower, except in so far as the lower partakes of something higher. Now it is evident that the form of a stone or of any sensible, is lower than man. Consequently the intellect is not perfected by the form of a stone, as such, but inasmuch as it partakes of a certain likeness to that which is above the human intellect, viz. the intelligible light, or something of the kind. Now whatever is by something else is reduced to that which is of itself. Therefore man's final perfection must needs be through knowledge of something above the human intellect. But it has been shown (I, 88, 2), that man cannot acquire through sensibles, the knowledge of separate substances, which are above the human intellect. Consequently it follows that man's happiness cannot consist in the consideration of speculative sciences. However, just as in sensible forms there is a participation of the higher substances, so the consideration of speculative sciences is a certain participation of true and perfect happiness.
Reply to Objection 1. In his book on Ethics the Philosopher treats of imperfect happiness, such as can be had in this life, as stated above (02, ad 4).
Reply to Objection 2. Not only is perfect happiness naturally desired, but also any likeness or participation thereof.
Reply to Objection 3. Our intellect is reduced to act, in a fashion, by the consideration of speculative sciences, but not to its final and perfect act.
Source: Summa Theologica (New Advent)