Of course there is such a thing as too much doubt, for we ought to accept what is true. But there is also such a thing as proper doubt, for we ought not accept what is false. The possibility of doubt is inherent in the longing to understand, and nothing less than complete and perfect knowledge can satisfy the mind. We do not possess such knowledge here on earth; it is reserved for the beatific vision. Until then, doubt will be with us. This is...why it is so unreasonable to trust only what cannot be doubted, as Descartes proposed, because everything can be doubted. We should believe, not what we cannot doubt, but what we have the best reasons to believe.
J. Budziszewski, Commentary on Thomas Aquinas's Treatise on Law
doubt, knowledge