Thomas Aquinas

“ Sometimes, however, desire taken in its proper sense, is the cause of sorrow. Because whatever hinders a movement from reaching its end is contrary to that movement. Now that which is contrary to the movement of the appetite, is a cause of sorrow. Consequently, desire becomes a cause of sorrow, in so far as we sorrow for the delay of a desired good, or for its entire removal. But it cannot be a universal cause of sorrow: since we sorrow more for the loss of present good, in which we have already taken pleasure, than for the withdrawal of future good which we desire to have. ”

“ Hence neither does the desire of any kind of unity cause pain or sorrow, as some have maintained: whose opinion is refuted by the Philosopher from the fact that repletion is not always pleasant; for instance, when a man has eaten to repletion, he takes no further pleasure in eating; because repletion or union of this kind, is repugnant rather than conducive to perfect being. Consequently sorrow is caused by the craving, not for any kind of unity, but for that unity in which the perfection of nature consists. ”

“ That which is desired, though really future, is, nevertheless, in a way, present, inasmuch as it is hoped for. Or we may say that although the desired good itself is future, yet the hindrance is reckoned as present, and so gives rise to sorrow. ”