Moral duty consists in the observance of those rules of conduct which
contribute to the welfare of society, and by implication, of the
individuals who compose it.
The end of society is peace and mutual protection, so that the
individual may reach the fullest and highest life attainable by man.
The rules of conduct by which this end is to be attained are
discoverable--like the other so-called laws of Nature--by observation
and experiment, and only in that way.
Some thousands of years of such experience have led to the
generalisations, that stealing and murder, for example, are inconsistent
with the ends of society. There is no more doubt that they are so than
that unsupported stones tend to fall. The man who steals or murders,
breaks his implied contract with society, and forfeits all protection.
He becomes an outlaw, to be dealt with as any other feral creature.
Criminal law indicates the ways which have proved most convenient for
dealing with him.
All this would be true if men had no "moral sense" at all, just as
there are rules of perspective which must be strictly observed by a
draughtsman, and are quite independent of his having any artistic sense.