this unity of principle, in that close union of
interest, which is so observable between the public and each individual.

But notwithstanding this frequent confusion of interests, it is easy
to attain what natural philosophers, after Lord Bacon, have affected to
call the experimentum crucis, or that experiment which points out the
right way in any doubt or ambiguity. We have found instances, in
which private interest was separate from public; in which it was
even contrary: And yet we observed the moral sentiment to continue,
notwithstanding this disjunction of interests. And wherever these
distinct interests sensibly concurred, we always found a sensible
increase of the sentiment, and a more warm affection to virtue, and
detestation of vice, or what we properly call, GRATITUDE and REVENGE.
Compelled by these instances, we must renounce the theory, which
accounts for every moral sentiment by the principle of self-love. We
must adopt a more public affection, and allow, that the interests of
society are not, even on their own account, entirely indifferent to
us. Usefulness is only a tendency to a certain end; and it is a
contradiction in terms, that anything pleases as means to an end, where
the end itself no wise affects us. If usefulness, therefore, be a source
of moral sentiment, and if this usefulness be not always considered with
a reference to self; it follows, that everything, which contributes to
the happiness of society, recommends itself directly to our approbation
and good-will. Here is a principle, which accounts, in great part, for
the origin of morality: And what need we seek for abstruse and remote
systems, when there occurs one so obvious and natural?

[FOOTNOTE: It is needless to push our researches so far as to ask, why
we have humanity or a fellow-feeling with others. It is sufficient,
that this is experienced to be a principle in human nature. We must stop
somewhere in our examination of causes; and there are, in every science,
some general principles, beyond which we cannot hope to find any
principle more general. No man is absolutely indifferent to the
happiness and misery of others. The first has a natural tendency to give
pleasure; the second, pain. This every one may find in himself. It is
not probable, that these principles can be resolved into principles
more simple and universal, whatever attempts may have been made to that
purpose. But if it were possible, it belongs not to the present subject;