Why is illegal contract a contradiction in terms

Extract

The term 'illegal contract' is sanctioned by usage and is adopted in the title of this article for the sake of brevity; but it is not a very satisfactory expression. If a contract is a legal obligation, 'illegal contract' is a contradiction in terms. To say that a contract is illegal seems, on the face of it, to be no more than a clumsy way of saying that an agreement is void of legal consequences. There is, however, another possible interpretation of the term 'illegal contract,' namely, that it is an agreement the entry into which or the performance of which (on one or both sides) involves a breach of legal duty, or runs counter to morality or public policy. The illegality is not in the obligation, but in the making of the agreement or in the performance of it. It is in this sense that the term is used in the present article, and, so understood, there is no paradox in asking whether an illegal contract can have a legal effect.