Objection 1. It would seem that doubt is a virtue. For I have doubts, and I am virtuous. Therefore, doubt is a virtue.
Objection 2. Further, people who don't have doubts are obnoxious. But being obnoxious is contrary to the virtue of charity. Therefore, doubt is a virtue.
Objection 3. Further, by doubting a man comes to accept and understand his faith more deeply. Since the fruits of doubt are good, doubt itself must be a virtue.
On the contrary, the Apostle writes, "For he that wavereth is like a wave of the sea, which is moved and carried about by the wind. Therefore let not that man think that he shall receive any thing of the Lord." [Jas. 1:6-7] Therefore doubt is not a virtue.
I answer that, doubt is contrary to faith, and whatever is contrary to a virtue cannot itself be a virtue.
But not all forms of doubt are so opposed to faith as to be vices. A man may doubt out of ignorance, as being unsure of something because he does not know that someone trustworthy has affirmed it, and this in itself is not sinful, if he cannot be blamed for his ignorance. Or a man may doubt out of a lack of clarity, as being unsure of the meaning of what a trustworthy person has affirmed, or out of an error in reasoning, as when he fails to see that a particular consequence necessarily follows from what he believes; in neither case is his doubt a sin in itself.
If, however, a man doubts through deliberately turning his will away from attending to the intellectual principle by which an object of faith is to be accepted, this may be accounted blindness of mind and a sin, as the Doctor writes. Further, a man may doubt through obstinately refusing to assent to that which is proposed as an article of faith, which is an act of unbelief and a sin.
Reply to Objection 1. Yeah, and so's my big toe.
Reply to Objection 2. Trust me, they'd be obnoxious even if they doubted.
Reply to Objection 3. The withholding of assent that is the act of doubt can be done in two ways. First, as an exercise of the intellect, whereby the content of faith is examined by asking such questions as, "What if it were not so?" This exercise is done with the purpose of deepening faith, and is not doubt properly so-called.
The second way, which is doubt proper, is to withhold assent unconditionally. This act terminates in a state in which the actor has less faith than before, and can in no way be held to cause an increase in faith. It may be that, subsequently, the man will grow in faith, but such growth requires other causes and cannot be regarded as the end for which the man doubts. If the man winds up with a greater faith than before he doubted, this is to be regarded as God bringing good out of evil, not of a virtuous means producing good fruit.