I have spoken of actions that we judge to be right and
that ought to be done as being ‘reasonable’ or ‘rational’, and
of ultimate ends as ‘prescribed by reason’; and I contrasted
the corresponding motives with ‘non-rational’ desires and
inclinations. This way of speaking is employed by writers of
various schools, and fits with ordinary language and ordinary
thought. We do commonly think that wrong conduct can
be shown by argument to be essentially irrational. We don’t
think that men are influenced to act rightly by reason alone,
but we hold that appeals to reason are an essential part of
all moral persuasion—the part that concerns the moralist
or moral philosopher as distinct from the preacher or moral
rhetorician. But many people think that, as Hume says,
‘Reason, meaning the judgment of truth and falsehood, can
never of itself be any motive to the will’, and that every
motive to action is some non-rational desire, taking ‘desires’
to include the impulses to action given by present pleasure
and pain. Before going further, we should look carefully at
the grounds of this contention.