Lord Mansfield stated his views of public policy in terms [204] not unlike those used by Chief Justice Holt in Coggs v.  Bernard, but distinctly confines their application to common carriers.  “But there is a further degree of responsibility by the custom of the realm, that is, by the common law; a carrier is in the nature of an insurer ....  To prevent litigation, collusion, and the necessity of going into circumstances impossible to be unravelled, the law presumes against the carrier, unless,” &c. 1

At the present day it is assumed that the principle is thus confined, and the discussion is transferred to the question who are common carriers.  It is thus conceded, by implication, that Lord Holt’s rule has been abandoned.  But the trouble is, that with it disappear not only the general system which we have seen that Lord Holt entertained, but the special reasons repeated by Lord Mansfield.  Those reasons apply to other bailees as well as to common carriers.  Besides, hoymen and masters of ships were not originally held because they were common carriers, and they were all three treated as co-ordinate species, even in Coggs v.  Bernard, where they were mentioned only as so many instances of bailees exercising a public calling.  We do not get a new and single principle by simply giving a single name to all the cases to be accounted for.  If there is a sound rule of public policy which ought to impose a special responsibility upon common carriers, as those words are now understood, and upon no others, it has never yet been stated.  If, on the other hand, there are considerations which apply to a particular class among those so designated,—­for instance, to railroads, who may have a private individual at their mercy, or exercise a power too vast for the common welfare,—­we do not prove that the [205] reasoning extends to a general ship or a public cab by calling all three common carriers.

If there is no common rule of policy, and common carriers remain a merely empirical exception from general doctrine, courts may well hesitate to extend the significance of those words.  Furthermore, notions of public policy which would not leave parties free to make their own bargains are somewhat discredited in most departments of the law. 1 Hence it may perhaps be concluded that, if any new case should arise, the degree of responsibility, and the validity and interpretation of any contract of bailment that there may be, should stand open to argument on general principles, and that the matter has been set at large so far as early precedent is concerned.

I have treated of the law of carriers at greater length than is proportionate, because it seems to me an interesting example of the way in which the common law has grown up, and, especially, because it is an excellent illustration of the principles laid down at the end of the first Lecture.  I now proceed to the discussion for the sake of which an account of the law of bailment was introduced, and to which an understanding of that part of the law is a necessary preliminary.