These words mean to make or attempt to make something certain or secure by removing doubt. Assure can mean either to make certain or to cause to feel certain.

• The doctor assured him that his child would recover from the illness; The fact that the hospital had a good reputation assured him that his child would be well taken care of.

Ensure , or, as it is also spelt, insure , means to make certain as the consequence of some action or agent: To ensure the child’s quick recovery, the doctor gave him an antibiotic. Ensure also means to make safe, to protect against harm; to ensure freedom against tyranny.

Guarantee means to assume responsibility for the quality of a product or for the performance of a service or obligation. One who guarantees a debt assures the creditor that he will be paid.

Promise , like assure , is often designed to make someone feel certain, but hardly guarantees that the outcome will measure up to one’s expectation. Promise implies intention, not obligation, and every child knows that not all promises are kept: to promise someone a rise in pay; to promise to keep an appointment. In another sense, promise simply indicates grounds for favourable expectation. An invalid’s renewed appetite may be deemed a promising sign.