Selection from St. Thomas Aquinas | De veritate, Q. 14, a. 1.
Trans. James V. McGlynn, S.J. (Chicago: Henry Regnery Company, 1953)

For, sometimes, it [the intellect] does not tend toward one rather than the other, either because of a lack of evidence, as happens in those problems about which we have no reasons for either side, or because of an apparent equality of the motives for both sides. This is the state of one in doubt, who wavers between the two members of a contradictory proposition.

Sometimes, however, the understanding tends more to one side than the other; still, that which causes the inclination does not move the understanding enough to determine it fully to one of the members. Under this influence, it accepts one member, but always has doubts about the other. This is the state of one holding an opinion, who accepts one member of the contradictory proposition with some fear that the other is true.

Sometimes, again, the possible intellect is so determined that it adheres to one member without reservation. This happens sometimes because of the intelligible object and sometimes because of the will. Furthermore, the intelligible object sometimes acts mediately, sometimes immediately. It acts immediately when the truth of the propositions is unmistakably clear immediately to the intellect from the intelligible objects themselves. This is the state of one who understands principles, which are known as soon as the terms are known, as the Philosopher says. Here, the very nature of the thing itself immediately determines the intellect to propositions of this sort. The intelligible object acts mediately, however, when the understanding, once it knows the definitions of the terms, is determined to one member of the contradictory proposition in virtue of first principles. This is the state of one who has science [knowledge].

Sometimes, however, the understanding can be determined to one side of a contradictory proposition neither immediately through the definitions of the terms, as is the case with principles, nor yet in virtue of principles, as is the case with conclusions from a demonstration. And in this situation our understanding is determined by the will, which chooses to assent to one side definitely and precisely because of something which is enough to move the will, though not enough to move the understanding, namely, since it seems good or fitting to assent to this side. And this is the state of one who believes. This may happen when someone believes what another says because it seems fitting or useful to do so.

Thus, too, we are moved to believe what God says because we are promised eternal life as a reward if we believe. And this reward moves the will to assent to what is said, although the intellect is not moved by anything which it understands. Therefore, Augustine says: “Man can do other things unwillingly, but he can believe only if he wills it.”