Perhaps the simplest and most important point about ethics is purely logical. I mean the impossibility to derive nonautological ethical rules—imperatives; principles of policy; aims; or however we may describe them—from statements of facts. Only if this fundamental logical problem is realized can we begin to formulate the real problems of moral philosophy, and to appreciate their difficulty.
Popper, K. Proceedings of the Aristotelian Society, supp. vol. XXII, 1948.