Doubt is a state of mind characterized by a feeling of uneasiness. Nevertheless, we cannot from a logical, and especially from a pragmaticistical point of view, regard the doubt as consisting in feeling. [—] A true doubt is […] a doubt which really interferes with the proper action of a belief-habit.
http://www.commens.org/dictionary/entry/quote-materials-monist-article-consequences-pragmaticism-vols-i-and-ii-r