Objection 1. It would seem that there cannot be, in the same power, specifically different passions that are not contrary to one another. For the passions of the soul differ according to their objects. Now the objects of the soul's passions are good and evil; and on this distinction is based the contrariety of the passions. Therefore no passions of the same power, that are not contrary to one another, differ specifically.
Objection 2. Further, difference of species implies a difference of form. But every difference of form is in respect of some contrariety, as stated in Metaph. x, 8. Therefore passions of the same power, that are not contrary to one another, do not differ specifically.
Objection 3. Further, since every passion of the soul consists in approach or withdrawal in respect of good or evil, it seems that every difference in the passions of the soul must needs arise from the difference of good and evil; or from the difference of approach and withdrawal; or from degrees in approach or withdrawal. Now the first two differences cause contrariety in the passions of the soul, as stated above (Article 2): whereas the third difference does not diversify the species; else the species of the soul's passions would be infinite. Therefore it is not possible for passions of the same power to differ in species, without being contrary to one another.
On the contrary, Love and joy differ in species, and are in the concupiscible power; and yet they are not contrary to one another; rather, in fact, one causes the other. Therefore in the same power there are passions that differ in species without being contrary to one another.
I answer that, Passions differ in accordance with their active causes, which, in the case of the passions of the soul, are their objects. Now, the difference in active causes may be considered in two ways: first, from the point of view of their species or nature, as fire differs from water; secondly, from the point of view of the difference in their active power. In the passions of the soul we can treat the difference of their active or motive causes in respect of their motive power, as if they were natural agents. For every mover, in a fashion, either draws the patient to itself, or repels it from itself. Now in drawing it to itself, it does three things in the patient. Because, in the first place, it gives the patient an inclination or aptitude to tend to the mover: thus a light body, which is above, bestows lightness on the body generated, so that it has an inclination or aptitude to be above. Secondly, if the generated body be outside its proper place, the mover gives it movement towards that place. Thirdly, it makes it to rest, when it shall have come to its proper place: since to the same cause are due, both rest in a place, and the movement to that place. The same applies to the cause of repulsion.
Now, in the movements of the appetitive faculty, good has, as it were, a force of attraction, while evil has a force of repulsion. In the first place, therefore, good causes, in the appetitive power, a certain inclination, aptitude or connaturalness in respect of good: and this belongs to the passion of "love": the corresponding contrary of which is "hatred" in respect of evil. Secondly, if the good be not yet possessed, it causes in the appetite a movement towards the attainment of the good beloved: and this belongs to the passion of "desire" or "concupiscence": and contrary to it, in respect of evil, is the passion of "aversion" or "dislike." Thirdly, when the good is obtained, it causes the appetite to rest, as it were, in the good obtained: and this belongs to the passion of "delight" or "joy"; the contrary of which, in respect of evil, is "sorrow" or "sadness."
On the other hand, in the irascible passions, the aptitude, or inclination to seek good, or to shun evil, is presupposed as arising from the concupiscible faculty, which regards good or evil absolutely. And in respect of good not yet obtained, we have "hope" and "despair." In respect of evil not yet present we have "fear" and "daring." But in respect of good obtained there is no irascible passion: because it is no longer considered in the light of something arduous, as stated above (Article 3). But evil already present gives rise to the passion of "anger."
Accordingly it is clear that in the concupiscible faculty there are three couples of passions; viz. love and hatred, desire and aversion, joy and sadness. In like manner there are three groups in the irascible faculty; viz. hope and despair, fear and daring, and anger which has not contrary passion.
Consequently there are altogether eleven passions differing specifically; six in the concupiscible faculty, and five in the irascible; and under these all the passions of the soul are contained.
From this the replies to the objections are evident.
Source: Summa Theologica (New Advent)