Certainty, certitude, assurance, conviction are comparable when denoting a state of mind in which one is free from doubt.
Certainty and certitude both imply the absence of all doubt as to the truth of something; they are not always distinguishable in use, although philosophers and psychologists have often tried to distinguish the states of mind which they designate. The psychological differentiation of certainty as the state of mind induced by something of which there is objectively as well as subjectively not the slightest question, from certitude as the state of mind of one whose faith or belief is so strong that it resists all attack, has indubitably affected the meanings and the use of these terms in general use (some philosophies tend to destroy man’s certainty of his own existence).
Certitude is usually more personal and less objective than certainty and suggests deeper roots for one’s freedom from doubt and less likelihood of a change of belief.
In its greater objectivity certainty also provides a term that may logically designate the quality of a thing believed.
Assurance (see also CONFIDENCE 2) stresses sureness and confidence rather than certainty; the grounds of such sureness need not be objective proofs or the evidence of one’s senses, for something of which one has assurance is typically something that is indemonstrable or is yet to happen; the word usually suggests implicit reliance on oneself or on one’s powers, one’s intuitions, or one’s methods or complete trust in another (as a source of information, a supporter, a sovereign, or God).
Conviction usually implies previous doubt or uncertainty. It involves the idea of certitude but is not its equivalent, for certitude may or may not imply a rational basis for one’s freedom from doubt, and conviction (see also OPINION) usually does. It differs from certainty in stressing one’s subjective reaction to evidence rather than the objective validity of the evidence itself. Conviction is therefore commonly applied to the state of mind of one who has been or is in the process of being convinced.