Man's natural desire for knowledge cannot terminate in any created -- corporeal or immaterial -- substance. This is because as man penetrates the essence of things, that is, as he acquires their intelligible species within himself, he must necessarily understand them as secondary things, effects. No desire for knowledge, though, is satiated through the knowledge of mere effects. The perfection of knowledge, then, consists in a knowledge of causes, but not the types of causes of which we have direct sensible experience. All causes of this sort are not causes simply but are instead causes of some things and effects of others. My father is, in a way, the cause of me while also being the effect of my grandfather. The human intellect cannot find rest in the knowledge of such causes but must come to know the first cause, the Uncaused Cause.
When man comes to know God, his natural desire for knowledge is fulfilled and, since God is the cause of all goodness, no further desire can remain within him. To "see" God as he is, through His own essence, is the culmination of human life and is known as beatitude.
In conclusion, all creatures attain their end through the generation of their form. The possession of this end is called goodness, and it is through creatures' individual goodness that they assimilate to the divine goodness. Man, the rational animal, does so in an utterly special way. As an intellectual creature, his form is most properly attained through the possession of a knowledge of that which is most true, good, and existent. Knowledge of God, Who is truth, goodness, and being itself, is the final cause of man since it is this knowledge which brings man into his respective perfection. Man's end culminates in beatitude.