Obj. 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 (A. 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.