1. To make someone feel certain that something is true Her arguments didn’t Convince everyone, but changes were made

2. Assure verb [T] (PROMISE) to promise or tell something to someone confidently or firmly, or to cause someone to feel certain by removing doubt: She Assured him (that) the check was in the mail

3. In his 1941 paper “ Certainty,” Moore observed that the word certain is commonly used in four main types of idiom: “I feel certain that,” “I am certain that,” “I know for certain that,” and “It is certain that.” He pointed out that there is at least one use of “I know for certain that p” and “It is certain that p” on which neither of those sentences can be true