I answer that, As the Philosopher says (Metaph. x, 4), contrariety is a difference in respect of a form.  Now a form may be generic or specific.  Consequently things may be contraries in respect of a generic form, as virtue and vice; or in respect of a specific form, as justice and injustice.

Now we must observe that some things are specified by absolute forms, e.g. substances and qualities; whereas other things are specified in relation to something extrinsic, e.g. passions and movements, which derive their species from their terms or objects.  Accordingly in those things that are specified by absolute forms, it happens that species contained under contrary genera are not contrary as to their specific nature:  but it does not happen for them to have any affinity or fittingness to one another.  For intemperance and justice, which are in the contrary genera of virtue and vice, are not contrary to one another in respect of their specific nature; and yet they have no affinity or fittingness to one another.  On the other hand, in those things that are specified in relation to something extrinsic, it happens that species belonging to contrary genera, are not only not contrary to one another, but also that they have a certain mutual affinity or fittingness.  The reason of this is that where there is one same relation to two contraries, there is contrariety; e.g. to approach to a white thing, and to approach to a black thing, are contraries; whereas contrary relations to contrary things, implies a certain likeness, e.g. to recede from something white, and to approach to something black.  This is most evident in the case of contradiction, which is the principle of opposition:  because opposition consists in affirming and denying the same thing, e.g. “white” and “non-white”; while there is fittingness and likeness in the affirmation of one contrary and the denial of the other, as, if I were to say “black” and “not white.”

Now sorrow and pleasure, being passions, are specified by their objects.  According to their respective genera, they are contrary to one another:  since one is a kind of pursuit, the other a kind of avoidance, which “are to the appetite, what affirmation and denial are to the intellect” (Ethic. vi, 2).  Consequently sorrow and pleasure in respect of the same object, are specifically contrary to one another:  whereas sorrow and pleasure in respect of objects that are not contrary but disparate, are not specifically contrary to one another, but are also disparate; for instance, sorrow at the death of a friend, and pleasure in contemplation.  If, however, those diverse objects be contrary to one another, then pleasure and sorrow are not only specifically contrary, but they also have a certain mutual fittingness and affinity:  for instance to rejoice in good and to sorrow for evil.

Reply Obj. 1:  Whiteness and blackness do not take their species from their relationship to something extrinsic, as pleasure and sorrow do:  wherefore the comparison does not hold.