Thomas Aquinas

“ 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 ”

“ 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. ”

“ For just as in the physical order, diversity of genus arises from diversity in the potentiality of matter, while diversity of species arises from diversity of form in the same matter; so in the acts of the soul, those that belong to different powers, differ not only in species but also in genus, while acts and passions regarding different specific objects, included under the one common object of a single power, differ as the species of that genus. ”

“ 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. ”