[This is from Arvin Vos’s interesting book “Aquinas, Calvin, & Contemporary Protestant Thought: A Critique of Protestant Views on the Thought of Thomas Aquinas” (Christian University Press, 1985), pp. 10-12]
According to Augustine, to believe is to think with assent (cum assentione cogitare). Aquinas fixes the meaning of this statement by noting that the verb to think (cogitare) refers not to just any act of intellectual knowing, but in the narrower sense to an act of intellectual consideration “that is accompanied by a certain searching prior to reaching complete understanding in the certitude of seeing.” Again, “in this more proper sense cogitatio describes the process of the mind searching before reaching its term in the full vision of the truth” (ST 2a2ae, 2, 1). It is, in other words, the state in which one is reasoning, puzzling, pondering, unable to come to a conclusion.
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In the same way that Aquinas associates belief with the state of puzzlement one experiences while searching for the answer to a question, he associates knowledge with the freedom from puzzlement that comes when one has finally found the answer to the question. Knoweldge for Aquinas entails the possession of a firm assent, free from pondering.
Given that belief involves a sort of pondering and does not cease, one might suppose that it is like two other states of mind–namely, doubting and having an opinion–since both of these states also involve a failure of the mind to reach a firm assent. To doubt is to be unsure about two or more alternatives so that one leans toward neither. To have an opinion is to lean tentatively toward one alternative, without being able to rule out others. According to Aquinas, however, faith is unlike both doubt and opinion because it involves certainty of a sort. Believers can have a firm certitude about what they believe even though they continue to ponder in a fashion similar to that of those who doubt or have an opinion:
“The act of believing . . . is firmly attached to one alternative and in this respect the believer is in the same state of mind as one who has science or understanding. Yet the believer’s knowledge is not completed by a clear vision, and in this respect he is like one having a doubt, a ssuspicion, or an opinion. To ponder with assent is, then distinctive of the believer” (ST 2a2ae, 2, 1).
The believer gives assent like one who has clear vision (i.e. like one who understands), but does so without having such vision. Naturally this raises a question: On what is the believer’s assent bassed if it is not based on understanding? Aquinas’s answer is that this assent is based on a voluntary choice:
“There are two ways in which the mind assents to anything. One way is by being actuated by the object to which it assents . . . The other way the mind assents is not through a sufficient motivation by its proper object, but through some voluntary choice that influences the mind in favor ofone alternative rather than the other” (ST, 2a2ae 1, 4).
When one’s understanding of an object does not provide a sufficient basis for assent, Aquinas says, the mind can assent by an act of the will. It is this sort of assent that constitutes faith.