It is the business of this life to make excuses for others, but none for ourselves. We should be clearly persuaded of our own misconduct, for that is the part of knowledge in which we are most apt to be defective.

Robert Louis Stevenson (1850-1894) Scottish essayist, novelist, poet

Essay (1880-01/02?), “Reflections and Remarks on Human Life,” § 1.1 “Justice and Justification”
    (Source)

A collection of aphorisms and musings, first published in the Edinburgh Edition of his Works, vol. 28 (1898).