Charles S. Peirce
Chapter 1: Of the Difference between Doubt & Belief
MS 187 (L 248): Writings 3, 22-23
Between 11 and 14 May 1872
We generally know when we wish to ask a question and when we wish to pronounce a judgment, for there is a dissimilarity between the sensation of doubting and that of believing.
But this is not all that distinguishes doubt and belief. There is a practical difference. Our beliefs guide our desires and shape our actions. The assassins or followers of the Old Man of the Mountain used to rush into death at his least command, because they believed that obedience to him would insure everlasting felicity. Had they doubted this, they would not have acted as they did. So it is with every belief according to its degree. The feeling of believing is a more or less sure indication of there being established in our nature something which will determine our actions. Doubt never has such an effect.
Nor must we overlook a third point of difference. Doubt is an uneasy and dissatisfied state from which we struggle to free ourselves and pass into the state of belief; while this latter is a calm and satisfactory state which we do not wish to avoid or to change to a belief in anything else. On the contrary we cling tenaciously not merely to believing but to believing just what we do believe.
Both doubt and belief have positive effects upon us, though very diverse ones. Belief does not make us act at once but puts us into such a condition that we shall behave in a certain way, when the occasion arises. Doubt has not the least effect of this sort, but stimulates us to try to destroy it. This reminds us of the irritation of a nerve and the reflex action produced thereby; while for the analogue of belief, in the nervous system, we must look to what are called nervous associations, for example to that habit of the nerves in consequence of which the smell of a peach will make the mouth water.