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Doubt is not below knowledge, but above it.

We know accurately only when we know little, with knowledge doubt increases.

Doubt is the middle position between knowledge and ignorance. It encompasses cynicism but also genuine questioning.

To believe with certainty we must begin with doubting.

Doubt is the incentive to truth and inquiry leads the way.

Doubt, the essential preliminary of all improvement and discovery, must accompany the stages of man's onward progress. The faculty of doubting and questioning, without which those of comparison and judgment would be useless, is itself a divine prerogative of the reason.

Doubt begins only at the last frontiers of what is possible.

Doubt is not a pleasant condition, but certainty is absurd.

It is easier to believe than to doubt.