Thomas Aquinas

“ I answer that, Happiness consists in an operation of the speculative rather than of the practical intellect. This is evident for three reasons. First because if man's happiness is an operation, it must needs be man's highest operation. Now man's highest operation is that of his highest power in respect of its highest object: and his highest power is the intellect, whose highest object is the Divine Good, which is the object, not of the practical but of the speculative intellect. Consequently happiness consists principally in such an operation, viz. in the contemplation of Divine things. ”

“ If therefore the human intellect, knowing the essence of some created effect, knows no more of God than "that He is"; the perfection of that intellect does not yet reach simply the First Cause, but there remains in it the natural desire to seek the cause. Wherefore it is not yet perfectly happy. Consequently, for perfect happiness the intellect needs to reach the very Essence of the First Cause. ”

“ For in God there is happiness essentially; since His very Being is His operation, whereby He enjoys no other than Himself. In the happy angels, the final perfection is in respect of some operation, by which they are united to the Uncreated Good: and this operation of theirs is one only and everlasting. But in men, according to their present state of life, the final perfection is in respect of an operation whereby man is united to God: but this operation neither can be continual, nor, consequently, is it one only, because operation is multiplied by being discontinued. ”