When a person of full intellectual maturity had deliberately bound himself by an engagement, he was said to be under a natural obligation, even though he had omitted some necessary formality, and even though through some technical impediment he was devoid of the formal capacity for making a valid contract. The law (and this is what the distinction implies) would not enforce the obligation, but it did not absolutely refuse to recognise it; and natural obligations differed in many respects from obligations which were merely null and void, more particularly in the circumstance that they could be civilly confirmed, if the capacity for contract were subsequently acquired.

Ancient Law, Sir Henry James Sumner Maine, 1917.