There are three kinds of necessity. One is that of constraint, which is wholly repugnant to the will; for violence is contrary to natural inclination. Another is that of Nature; and this accords with the will: for all desire Beatitude, which is the inherent end of the will, and stands towards it as first principles to the intellect. The third kind of necessity is that of supposition, where an end can only be reached in one way, such as the necessity of a ship for crossing the sea; this also accords with the will.
But the will does not choose everything by necessity; for good being the final end, those things only upon which it actually depends are willed of necessity. Thus the intellect apprehends first principles and those things which have a necessary connection with them, - such as demonstrable conclusions once they are known, - but not contingent conclusions, which may be denied without affecting the premises. And in like manner the will is not compelled to desire particular goods which have no necessary connection with Beatitude, since the latter can be attained without them.
The intellect considered in itself is, absolutely speaking, a higher faculty than the will, although relatively the will is sometimes to be preferred. For the proper order of the faculties follows that of their relation to the object; and since the object of the intellect is this very conception of desirable good, it is simpler and more absolute than the desirable good which is the object of the will. Relatively, however, the will may be nobler; as when the desired object is superior to the soul itself, hence it is better to love God than to know Him. For the act of the intellect consists in that conception of the object which is in the intelligent mind; while the act of the will is directed to the object as it exists in itself Thus the intellect moves the will by mode of end; for the end moves the efficient; and intellectual good, being the object of the will, moves it as the end; while the will moves the intellect by mode of agent, as that which alters moves the thing altered, or the impeller the impelled. And, in all regulated faculties, those which regard the particular end are subject to those which regard the universal end; excepting, however, the faculties which belong to the vegetative part, which are not subject to the will.
The will is not divided by the irascible and concupiscible faculties; because faculties which are ordered to good according to the general conception of it are not diversified by special differences. As with sight in relation to colour, the visual faculty is not multiplied according to the diversity of colours, so the will being related to good under a general conception is not diversified by the distinctions of the irascible and the concupiscible.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni