A belief is chiefly an affair of the soul, not of the consciousness; a doubt, on the contrary, is chiefly an affair of consciousness. It is an uneasy feeling, a special condition of irritation, in which the idea of two incompatible modes of conduct [is] before the doubter’s imagination, and nothing determines him, indeed he feels himself forbidden, to adopt either and reject the other. Of course it is not necessary that the degrees of dissatisfaction with the opposite alternatives should be equal. Like irritations generally, doubt sets up a reaction which does not cease until the irritation is removed. If we accept this account of the matter, doubt is not the direct negation or contrary of belief; for the two mainly affect different parts of the man. Speaking physiologically, belief is a state of the connections between different parts of the brain, doubt an excitation of brain-cells. Doubt acts quite promptly to destroy belief. Its first effect is to destroy the state of satisfaction. Yet the belief-habit may still subsist. But imagination so readily affects this habit, that the former believer will soon begin to act in a half-hearted manner, and before long the habit will be destroyed. The most important character of doubt is that no sooner does a believer learn that another man equally well- informed and equally competent doubts what he has believed, than he begins by doubting it himself. Probably the first symptom of this state of irritation will be anger at the other man. Such anger is a virtual acknowledgment of one’s own doubt; that is to say, not a genuine doubt, or feeling of uneasiness, but a sense that it is possible we may come to doubt it. Such doubt, at first of a purely external nature, sets up as reaction an effort to enter into the doubt and to comprehend it. Indeed, it is not necessary that one should actually meet with a man who doubts; for such is the influence of imagination in such matters that as soon as a believer can imagine that a man, equally well-informed and equally competent with himself, should doubt, doubt actually begins to set in, in his own state of feeling. From this follows the important corollary that if a man does not himself really doubt a given proposition he cannot imagine how it can be doubted, and therefore cannot produce any argument tending to allay such doubt. It thus appears that it is one thing to question a proposition and quite another to doubt it. We can throw any proposition into the interrogative mood at will; but we can no more call up doubt than we can call up the feeling of hunger at will. What one does not doubt one cannot doubt, and it is only accidentally that attention can be drawn to it in a manner which suggests the idea that there might be a doubt. Thence comes a critical attitude, and finally, perhaps, a genuine doubt may arise.