Sir John W. Salmond, Jurisprudence (1913)
“ Custom, merely as such, has no legal authority at all; it is legally effective only because and in so far as it is the expression of an underlying principle of right approved by those who use it. When it is based on no such ethical conviction or opinio necessitatis—when those who use it hold themselves free to depart from it if they will—it is of no legal significance. The only customs which are a source of law are those which are observed by the community as determining the rights and duties of its members. ”
