A cheerful, easy countenance and behavior are very useful: they make fools think you a good-natured man, and they make designing men think you an undesigning one.

Lord Chesterfield (1694-1773) English statesman, wit [Philip Dormer Stanhope]
Letter to his son, #298, enclosed maxims (15 Jan 1758)
    (Source)

Labeled as letter #297 in the linked source, but #298 in the volume I am using as reference, which does not include the maxims.