Reply Obj. 1: The inclination of the appetite to the possession of good causes the inclination of the appetite to fly from evil, as stated above. And hence it is that the appetitive movements that regard good, are reckoned as causing the appetitive movements that regard evil.
Reply Obj. 2: 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.
Reply Obj. 3: Desire gives pleasure, so long as there is hope of obtaining that which is desired. But, when hope is removed through the presence of an obstacle, desire causes sorrow. ________________________
THIRD ARTICLE [I-II, Q. 36, Art. 3]
Whether the Craving for Unity Is a Cause of Sorrow?
Objection 1: It would seem that the craving for unity is not a cause of sorrow. For the Philosopher says (Ethic. x, 3) that "this opinion," which held repletion to be the cause of pleasure, and division [*Aristotle wrote endeian, 'want'; St. Thomas, in the Latin version, read 'incisionem'; should he have read 'indigentiam'?], the cause of sorrow, "seems to have originated in pains and pleasures connected with food." But not every pleasure or sorrow is of this kind. Therefore the craving for unity is not the universal cause of sorrow; since repletion pertains to unity, and division is the cause of multitude.
Obj. 2: Further, every separation is opposed to unity. If therefore sorrow were caused by a craving for unity, no separation would be pleasant: and this is clearly untrue as regards the separation of whatever is superfluous.
Obj. 3: Further, for the same reason we desire the conjunction of good and the removal of evil. But as conjunction regards unity, since it is a kind of union; so separation is contrary to unity. Therefore the craving for unity should not be reckoned, rather than the craving for separation, as causing sorrow.
On the contrary, Augustine says (De Lib. Arb. iii, 23), that "from the pain that dumb animals feel, it is quite evident how their souls desire unity, in ruling and quickening their bodies. For what else is pain but a feeling of impatience of division or corruption?"
I answer that, Forasmuch as the desire or craving for good is reckoned as a cause of sorrow, so must a craving for unity, and love, be accounted as causing sorrow. Because the good of each thing consists in a certain unity, inasmuch as each thing has, united in itself, the elements of which its perfection consists: wherefore the Platonists held that one is a principle, just as good is. Hence everything naturally desires unity, just as it desires goodness: and therefore, just as love or desire for good is a cause of sorrow, so also is the love or craving for unity.