Accordingly, if "to think" be understood broadly according to the
first sense, then "to think with assent," does not express completely
what is meant by "to believe": since, in this way, a man thinks with
assent even when he considers what he knows by science [*Science is
certain knowledge of a demonstrated conclusion through its
demonstration.], or understands. If, on the other hand, "to think" be
understood in the second way, then this expresses completely the
nature of the act of believing. For among the acts belonging to the
intellect, some have a firm assent without any such kind of thinking,
as when a man considers the things that he knows by science, or
understands, for this consideration is already formed. But some acts
of the intellect have unformed thought devoid of a firm assent,
whether they incline to neither side, as in one who "doubts"; or
incline to one side rather than the other, but on account of some
slight motive, as in one who "suspects"; or incline to one side yet
with fear of the other, as in one who "opines." But this act "to
believe," cleaves firmly to one side, in which respect belief has
something in common with science and understanding; yet its knowledge
does not attain the perfection of clear sight, wherein it agrees with
doubt, suspicion and opinion. Hence it is proper to the believer to
think with assent: so that the act of believing is distinguished from
all the other acts of the intellect, which are about the true or the
false.

Reply Obj. 1: Faith has not that research of natural reason which
demonstrates what is believed, but a research into those things
whereby a man is induced to believe, for instance that such things
have been uttered by God and confirmed by miracles.