A fluctuation of mind arising from defect of knowledge or evidence; uncertainty of judgment or mind; unsettled state of opinion concerning the reality of an event, or the truth of an assertion, etc.; hesitation. "Doubt is the beginning and the end of our efforts to know." "Doubt, in order to be operative in requiring an acquittal, is not the want of perfect certainty (which can never exist in any question of fact) but a defect of proof preventing a reasonable assurance of quilt."