rational end the principle of his actions, while at the same time he
joins with this the necessary knowledge which, as it aims at action,
must not be spun out into the most subtile threads of metaphysic,
unless a legal duty is in question; in which case meum and tuum must
be accurately determined in the balance of justice, on the principle
of equality of action and action, which requires something like
mathematical proportion, but not in the case of a mere ethical duty.
For in this case the question is not only to know what it is a duty to
do (a thing which on account of the ends that all men naturally have
can be easily decided), but the chief point is the inner principle
of the will namely that the consciousness of this duty be also the
spring of action, in order that we may be able to say of the man who
joins to his knowledge this principle of wisdom that he is a practical
philosopher.



Now in this philosophy (of ethics) it seems contrary to the idea
of it that we should go back to metaphysical elements in order to make
the notion of duty purified from everything empirical (from every
feeling) a motive of action. For what sort of notion can we form of
the mighty power and herculean strength which would be sufficient to
overcome the vice-breeding inclinations, if Virtue is to borrow her
"arms from the armoury of metaphysics," which is a matter of
speculation that only few men can handle? Hence all ethical teaching
in lecture rooms, pulpits, and popular books, when it is decked out
with fragments of metaphysics, becomes ridiculous. But it is not,
therefore, useless, much less ridiculous, to trace in metaphysics
the first principles of ethics; for it is only as a philosopher that
anyone can reach the first principles of this conception of duty,