legal systems by derivation from the Roman books. Roman law may have a contractual conception of obligation _ex delicto_--thinking of the delict as giving rise to a debt--and the common law a delictual conception of liability upon contract--thinking in terms of recovery of damages for the wrong of breaking a promise--without much difference in the ultimate results. The fundamental things are not tort and contract but justifiable assumptions as to the mode in which one's fellow men will act in civilized society in many different situations of which aggression and undertaking are but two common types. Returning to our second postulate of due care in affirmative courses of conduct, we may note that in the society of today it is no less fundamental than the postulate of no intentional aggression. Aggression is the chief if not the only form of anti-social conduct in a primitive society. Indeed, a Greek writer on law and politics of the fifth century B. C. knew of no other subject of legal precepts. But with the development of machinery and consequent increase in human powers of action, the general security comes to be threatened quite as much by the way in which one does things as by what he does. Carelessness becomes a more frequent and more serious source of danger to the general security than aggression. Hence a set of nominate delicts requiring _dolus_ is supplemented by a theory of _culpa_. Hence a set of nominate torts, characterized by intentional aggression, is supplemented by liability for negligence, and the latter becomes the more important source of legal liability in practice.