The original principles of liability for harm inflicted by [5] another person or thing have been less carefully considered hitherto than those which governed trespass, and I shall therefore devote the rest of this Lecture to discussing them.  I shall try to show that this liability also had its root in the passion of revenge, and to point out the changes by which it reached its present form.  But I shall not confine myself strictly to what is needful for that purpose, because it is not only most interesting to trace the transformation throughout its whole extent, but the story will also afford an instructive example of the mode in which the law has grown, without a break, from barbarism to civilization.  Furthermore, it will throw much light upon some important and peculiar doctrines which cannot be returned to later.

A very common phenomenon, and one very familiar to the student of history, is this.  The customs, beliefs, or needs of a primitive time establish a rule or a formula.  In the course of centuries the custom, belief, or necessity disappears, but the rule remains.  The reason which gave rise to the rule has been forgotten, and ingenious minds set themselves to inquire how it is to be accounted for.  Some ground of policy is thought of, which seems to explain it and to reconcile it with the present state of things; and then the rule adapts itself to the new reasons which have been found for it, and enters on a new career.  The old form receives a new content, and in time even the form modifies itself to fit the meaning which it has received.  The subject under consideration illustrates this course of events very clearly.

I will begin by taking a medley of examples embodying as many distinct rules, each with its plausible and seemingly sufficient ground of policy to explain it.

[6] A man has an animal of known ferocious habits, which escapes and does his neighbor damage.  He can prove that the animal escaped through no negligence of his, but still he is held liable.  Why?  It is, says the analytical jurist, because, although he was not negligent at the moment of escape, he was guilty of remote heedlessness, or negligence, or fault, in having such a creature at all.  And one by whose fault damage is done ought to pay for it.

A baker’s man, while driving his master’s cart to deliver hot rolls of a morning, runs another man down.  The master has to pay for it.  And when he has asked why he should have to pay for the wrongful act of an independent and responsible being, he has been answered from the time of Ulpian to that of Austin, that it is because he was to blame for employing an improper person.  If he answers, that he used the greatest possible care in choosing his driver, he is told that that is no excuse; and then perhaps the reason is shifted, and it is said that there ought to be a remedy against some one who can pay the damages, or that such wrongful acts as by ordinary human laws are likely to happen in the course of the service are imputable to the service.