The Consensual Contracts, it will be observed, were extremely limited in number. But it cannot be doubted that they constituted the stage in the history of Contract-law from which all modern conceptions of contract took their start. The motion of the will which constitutes agreement was now completely insulated, and became the subject of separate contemplation; forms were entirely eliminated from the notion of contract, and external acts were only regarded as symbols of the internal act of volition. The Consensual Contracts had, moreover, been classed in the Jus Gentium, and it was not long before this classification drew with it the inference that they were the species of agreement which represented the engagements approved of by Nature and included in her code. This point once reached, we are prepared for several celebrated doctrines and distinctions of the Roman lawyers. One of them is the distinction between Natural and Civil Obligations. 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. Another very peculiar doctrine of the jurisconsults could not have had its origin earlier than the period at which the Convention was severed from the technical ingredients of Contract. They taught that though nothing but a Contract could be the foundation of an _action_, a mere Pact or Convention could be the basis of a _plea_. It followed from this, that though nobody could sue upon an agreement which he had not taken the precaution to mature into a Contract by complying with the proper forms, nevertheless a claim arising out of a valid contract could be rebutted by proving a counter-agreement which had never got beyond the state of a simple convention. An action for the recovery of a debt could be met by showing a mere informal agreement to waive or postpone the payment.