I answer that, The passions of the irascible part differ in species from those of the concupiscible faculty. For since different powers have different objects, as stated in the I:77:3, the passions of different powers must of necessity be referred to different objects. Much more, therefore, do the passions of different faculties differ in species; since a greater difference in the object is required to diversify the species of the powers, than to diversify the species of passions or actions. 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.
In order, therefore, to discern which passions are in the irascible, and which in the concupiscible, we must take the object of each of these powers. For we have stated in the I:81:2, that the object of the concupiscible power is sensible good or evil, simply apprehended as such, which causes pleasure or pain. But, since the soul must, of necessity, experience difficulty or struggle at times, in acquiring some such good, or in avoiding some such evil, in so far as such good or evil is more than our animal nature can easily acquire or avoid; therefore this very good or evil, inasmuch as it is of an arduous or difficult nature, is the object of the irascible faculty. Therefore whatever passions regard good or evil absolutely, belong to the concupiscible power; for instance, joy, sorrow, love, hatred, and such like: whereas those passionswhich regard good or bad as arduous, through being difficult to obtain or avoid, belong to the irascible faculty; such are daring, fear, hope and the like.
ST I-II Q. 23 A. 1