The most famous attempt to convey in summary fashion
their essential difference is the theory which asserts that, while
legal rules only require 'external' behaviour and are indifferent to the motives, intentions, or other 'internal' accompaniments
of conduct, morals on the other hand do not require
any specific external actions but only a good will or proper
intentions or motive. This really amounts to the surprising
assertion that legal and moral rules properly understood could
not ever have the same content; and though it does contain
a hint of the truth it is, as it stands, profoundly misleading.
It is in fact an inference, though a mistaken one, from certain
important characteristics of morals, and particularly from
certain differences between moral blame and legal punishment.
If someone does something forbidden by moral rules or
fails to do what they require, the fact that he did so unintentionally
and in spite of every care is an excuse from moral
blame; whereas a legal system or custom may have rules of
'strict liability' under which those who have broken the rules
unintentionally and without 'fault' may be liable to punishment.
So it is indeed true that while the notion of 'strict
liability' in morals comes as near to being a contradiction in
terms as anything in this sphere, it is something which may
be merely open to criticism when found in a legal system. But
this does not mean that morals require only good intention,
will, or motives. Indeed to argue thus is, as we show later, to
confuse the idea of an excuse with that of a justification for
conduct.