is often made the basis of
liability. But here the basis of liability must be found in a
relation. The fiction of an undertaking to use the skill or diligence
involved in a relation or calling is a juristic way of saying that one
who deals with another in such a relation or with another who
professes such a calling is justified in assuming the skill and
diligence ordinarily involved therein, so that the law holds those in
the relation or engaged in the calling to that standard in order to
maintain the general security. In other words another, though closely
related, postulate of civilized society is involved.

It is worth a moment's digression to suggest that such things show how
little the historical categories of delict and contract represent any
essential or inherent need of legal thinking. Austin thought that "the
distinction of obligations (or of duties corresponding to rights
against persons specifically determined) into obligations which arise
from contracts, obligations which arise from injuries, and obligations
which arise from incidents which are neither contracts nor injuries,"
was a "necessary distinction," without which a "system of law evolved
in a refined community" could not be conceived. This "necessary"
systematic scheme, which must be "a constituent part" of any
imaginable developed legal system, is but the Roman division into
obligations _ex contractu_, obligations _ex delicto_ and obligations
_ex uariis causarum figuris_, in which the third category is obviously
a catch-all. In trying to fit our law into this necessary scheme, we
find three types of cases must go in the third: (a) Duties or
liabilities attached by law to a relation, (b) duties imposed by law
to prevent unjust enrichment, (c) duties involved in an office or
calling. In the third of these our Anglo-American procedure allows
recovery either _ex delicto_ or _ex contractu_. In the second our law
sometimes goes on a property theory of constructive trust. In the
first duties are sometimes sanctioned affirmatively by conferring
legal powers or negatively by legal non-restraint of natural powers,
as in the law of domestic relations, where the wife has a power to
pledge the husband's credit for necessaries and the law does not
interfere with the parent's administering reasonable "correction" to
the child. Are we to say that these dogmatic departures of our law
from the Roman scheme are inconceivable or that because of them our
law is not ma