If I may be allowed to use the word “Habit” to denote any state of mind by virtue of which a person would, under definite circumstances, — mostly, if not invariably, consisting in his experiencing conscious experience of some kind, — either think, or act, or feel in a definite way, […] then doubt may be defined as the state of mind in which one is stimulated to incompossible intellectual assents.
http://www.commens.org/dictionary/entry/quote-logical-critique-essential-articles-religious-faith-6