Most of these distinctions originate in the ancient civil, and praetorian Roman law. Though some of them are not strictly founded in reason and truth. But the most obvious and natural way of discovering the truth is by referring to laws, which derive their force and efficacy from the general consent of mankind; so that if a law rests upon the presumption of any fact, which in reality has no existence, such a law is not binding. For when no evidence of the fact can be produced, the entire foundation, on which that law rests must fail. But we must have recourse to the subject, to the words and circumstances of a law, to determine when it is founded on such a presumption.[27] The same rule applies to the interpretation of promises. For where they are made upon the supposition of a fact, which in the end proves not to be true, they lose the force of obligations. Because the promiser made them upon certain conditions only, the fulfilment of which becomes impossible. Cicero, in his first book on the talents and character of an orator, puts the case of a father, who, under the supposition or intelligence that his son was dead, promised to devise his property to his nephew. But the supposition proving erroneous, and the intelligence false, the father was released from the obligation of the promise made to his relative. But if the promiser has neglected to examine the matter, or has been careless in expressing his meaning, he will be bound to repair the damage which another has sustained on that account. This obligation is not built on the strength of the promise, but on the injury, which it has occasioned. An erroneous promise will be binding, if the error was not the OCCASION of the promise. For here there is no want of consent in the party, who made it. But if the promise was obtained by fraud, the person so obtaining it shall indemnify the promiser for the injury sustained, if there has been any partial error in the promise, yet in other respects it shall be deemed valid. VII. Promises extorted by fear are a subject of no less intricate decision. For here too a distinction is usually made between a well founded and a chimerical fear, between a just fear and a bare suspicion, and between the persons who occasion it, whether it be the person to whom the promise is given, or some other. A distinction is also made between acts purely gratuitous, and those in which both parties have an interest. For according to all this variety of circumstances some engagements are considered as void, others as revocable at the pleasure or discretion of the maker, and others as warranting a claim to indemnity for the inconvenience occasioned. But on each of these points there is great diversity of opinion.