It is therefore clear that legal responsibility is not necessarily
excluded by the demonstration that an accused person could not have kept the law which he has broken; by contrast,
in morals 'I could not help it' is always an excuse, and
moral obligation would be altogether different from what it is
if the moral 'ought' did not in this sense imply 'can'. Yet it
is important to see that 'I could not help it' is only an excuse
(though a good one), and to distinguish excuse from justification;
for, as we have said, the claim that morals do not
require external behaviour rests on a confusion of these two
ideas. If good intentions were a justification for doing what
moral rules forbid, there would be nothing to deplore in the
action of a man who had accidentally and in spite of every
care killed another. We should look upon it as we now look
upon a man's killing another, when this is required as a
necessary measure of self-defence. The latter is justified because
killing, in such circumstances, is a kind of conduct which the
system is not concerned to prevent and may even encourage,
though it is of course an exception to a general prohibition of
killing. Where someone is excused because he offended unintentionally,
the underlying moral conception is not that this
action is of a kind which it is the policy of the law to permit
or even welcome; it is that when we investigate the mental
condition of the particular offender, we find that he lacked
the normal capacity to conform to the law's requirements.
Hence this aspect of the 'internality' of morals does not mean
that morals is not a form of control of outward conduct; but
only that it is a necessary condition for moral responsibility
that the individual must have a certain type of control over
his conduct. Even in morals there is a difference between 'He
did not do the wrong thing' and 'He could not help doing
what he did'.