I shall use the history of our law so far as it is necessary to explain a conception or to interpret a rule, but no further. In doing so there are two errors equally to be avoided both by writer and reader. One is that of supposing, because an idea seems very familiar and natural to us, that it has always been so. Many things which we take for granted have had to be laboriously fought out or thought out in past times. The other mistake is the opposite one of asking too much of history. We start with man full grown. It may be assumed that the earliest barbarian whose practices are to be considered, had a good many of the same feelings and passions as ourselves. The first subject to be discussed is the general theory of liability civil and criminal. The Common Law has changed a good deal since the beginning of our series of reports, and the search after a theory which may now be said to prevail is very much a study of tendencies. I believe that it will be instructive to go back to the early forms of liability, and to start from them. It is commonly known that the early forms of legal procedure were grounded in vengeance. Modern writers [3] have thought that the Roman law started from the blood feud, and all the authorities agree that the German law begun in that way. The feud led to the composition, at first optional, then compulsory, by which the feud was bought off. The gradual encroachment of the composition may be traced in the Anglo-Saxon laws, /1/ and the feud was pretty well broken up, though not extinguished, by the time of William the Conqueror. The killings and house-burnings of an earlier day became the appeals of mayhem and arson. The appeals de pace et plagis and of mayhem became, or rather were in substance, the action of trespass which is still familiar to lawyers. /2/ But as the compensation recovered in the appeal was the alternative of vengeance, we might expect to find its scope limited to the scope of vengeance. Vengeance imports a feeling of blame, and an opinion, however distorted by passion, that a wrong has been done. It can hardly go very far beyond the case of a harm intentionally inflicted: even a dog distinguishes between being stumbled over and being kicked.