Reply Obj. 1: Not every kind of union causes perfect goodness, but only that on which the perfect being of a thing depends. 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.
Reply Obj. 2: Separation can be pleasant, either because it removes something contrary to a thing's perfection, or because it has some union connected with it, such as union of the sense to its object.
Reply Obj. 3: Separation from things hurtful and corruptive is desired, in so far as they destroy the unity which is due. Wherefore the desire for such like separation is not the first cause of sorrow, whereas the craving for unity is. ________________________
FOURTH ARTICLE [I-II, Q. 36, Art. 4]
Whether an Irresistible Power Is a Cause of Sorrow?
Objection 1: It would seem that a greater power should not be reckoned a cause of sorrow. For that which is in the power of the agent is not present but future. But sorrow is for present evil. Therefore a greater power is not a cause of sorrow.
Obj. 2: Further, hurt inflicted is the cause of sorrow. But hurt can be inflicted even by a lesser power. Therefore a greater power should not be reckoned as a cause of sorrow.
Obj. 3: Further, the interior inclinations of the soul are the causes of the movements of appetite. But a greater power is something external. Therefore it should not be reckoned as a cause of sorrow.
On the contrary, Augustine says (De Nat. Boni xx): "Sorrow in the soul is caused by the will resisting a stronger power: while pain in the body is caused by sense resisting a stronger body."
I answer that, As stated above (A. 1), a present evil, is cause of sorrow or pain, by way of object. Therefore that which is the cause of the evil being present, should be reckoned as causing pain or sorrow. Now it is evident that it is contrary to the inclination of the appetite to be united with a present evil: and whatever is contrary to a thing's inclination does not happen to it save by the action of something stronger. Wherefore Augustine reckons a greater power as being the cause of sorrow.