Lon L. Fuller said this quote

“ What I have called the internal morality of law is in this sense a procedural version of natural law, though to avoid misunderstanding the word procedural should be assigned a special and expanded sense so that it would include, for example, a substantive accord between official action and enacted law. The term procedural is, however, broadly appropriate as indicating that we are concerned, not with the substantive aims of legal rules, but with the ways in which a system of rules for governing human conduct must be constructed and administered if it is to be efficacious and at the same time remain what it purports to be. ”