Objection 1. It would seem that the contrariety of the irascible passions is based on no other contrariety than that of good and evil. For the irascible passions are ordained to the concupiscible passions, as stated above (01, ad 1). But the contrariety of the concupiscible passions is no other than that of good and evil; take, for instance, love and hatred, joy and sorrow. Therefore the same applies to the irascible passions.
Objection 2. Further, passions differ according to their objects; just as movements differ according to their termini. But there is no other contrariety of movements, except that of the termini, as is stated in Phys. v, 3. Therefore there is no other contrariety of passions, save that of the objects. Now the object of the appetite is good or evil. Therefore in no appetitive power can there be contrariety of passions other than that of good and evil.
Objection 3. Further, "every passion of the soul is by way of approach and withdrawal," as Avicenna declares in his sixth book of Physics. Now approach results from the apprehension of good; withdrawal, from the apprehension of evil: since just as "good is what all desire" (Ethic. i, 1), so evil is what all shun. Therefore, in the passions of the soul, there can be no other contrariety than that of good and evil.
On the contrary, Fear and daring are contrary to one another, as stated in Ethic. iii, 7. But fear and daring do not differ in respect of good and evil: because each regards some kind of evil. Therefore not every contrariety of the irascible passions is that of good and evil.
I answer that, Passion is a kind of movement, as stated in Phys. iii, 3. Therefore contrariety of passions is based on contrariety of movements or changes. Now there is a twofold contrariety in changes and movements, as stated in Phys. v, 5. One is according to approach and withdrawal in respect of the same term: and this contrariety belongs properly to changes, i.e. to generation, which is a change "to being," and to corruption, which is a change "from being." The other contrariety is according to opposition of termini, and belongs properly to movements: thus whitening, which is movement from black to white, is contrary to blackening, which is movement from white to black.
Accordingly there is a twofold contrariety in the passions of the soul: one, according to contrariety of objects, i.e. of good and evil; the other, according to approach and withdrawal in respect of the same term. In the concupiscible passions the former contrariety alone is to be found; viz. that which is based on the objects: whereas in the irascible passions, we find both forms of contrariety. The reason of this is that the object of the concupiscible faculty, as stated above (Article 1), is sensible good or evil considered absolutely. Now good, as such, cannot be a term wherefrom, but only a term whereto, since nothing shuns good as such; on the contrary, all things desire it. In like manner, nothing desires evil, as such; but all things shun it: wherefore evil cannot have the aspect of a term whereto, but only of a term wherefrom. Accordingly every concupiscible passion in respect of good, tends to it, as love, desire and joy; while every concupiscible passion in respect of evil, tends from it, as hatred, avoidance or dislike, and sorrow. Wherefore, in the concupiscible passions, there can be no contrariety of approach and withdrawal in respect of the same object.
On the other hand, the object of the irascible faculty is sensible good or evil, considered not absolutely, but under the aspect of difficulty or arduousness. Now the good which is difficult or arduous, considered as good, is of such a nature as to produce in us a tendency to it, which tendency pertains to the passion of "hope"; whereas, considered as arduous or difficult, it makes us turn from it; and this pertains to the passion of "despair." In like manner the arduous evil, considered as an evil, has the aspect of something to be shunned; and this belongs to the passion of "fear": but it also contains a reason for tending to it, as attempting something arduous, whereby to escape being subject to evil; and this tendency is called "daring." Consequently, in the irascible passions we find contrariety in respect of good and evil (as between hope and fear): and also contrariety according to approach and withdrawal in respect of the same term (as between daring and fear).
From what has been said the replies to the objections are evident.
Source: Summa Theologica (New Advent)