Letters 94.10

“Shall you then offer precepts that are clear, or precepts that are doubtful? Those which are clear need no counsellor, and doubtful precepts gain no credence; so the giving of precepts is superfluous. That this is so learn thus: if you are counselling someone on a matter which is of doubtful clearness and doubtful meaning, you must supplement your precepts by proofs; and if you must resort to proofs, your means of proof are more effective and more satisfactory in themselves.

Seneca, Letters to Lucilius 94.10, translated by Richard Mott Gummere