Every satisfactory definition of law implies a sanction. Some penalty must be imposed upon a law-breaker, to be exacted, in the last resort, by external power. Force, therefore, is vital to law as it is to war, though normally it plays a less obvious part. A felon who is brought up for trial, condemned, and sent to prison, is induced by force, or by the fear of force, to submit to the court and to punishment. The policeman and the warder are the instruments of external power by which he is constrained to obey.
Force also supplies the most important incentive for securing obedience to law. It is true, as Maine pointed out, that for every man who keeps the law through conscious fear of punishment, there may be hundreds who do so as it were instinctively, and without a thought on the subject. But while this law-abiding spirit, which is characteristic of large sections of a modern community, owes its origin to a number of causes, perhaps the most potent of all has been the enforcement of law through long ages in the past.