“If you do not work on an important problem, it's unlikely you'll do important work.”
Richard Hamming, You and Your Research (1986) · Bell Communications Research colloquium, 7 March 1986 · read it
Kept because it is obvious and almost nobody acts on it. Hamming's test was to ask colleagues at lunch what the important problems in their field were, then why they were not working on one. Most could not answer the second question.