system of moral hygiene, which a man may adopt or not: only, like any
other physician, the professor of Ethics utters a friendly warning
that misery must ensue upon the neglect of what makes for health.

Deontology, not Ethics, expounds and vindicates the idea, _I ought_.
It is the science of Duty. It carries the mild suasions of Ethics into
laws, and out of moral prudence it creates conscience. And whereas
Ethics do not deal with sin, except under the aspect of what is called
"philosophical sin" (p. 119, S 6), Deontology defines sin in its
proper theological sense, as "an offence against God, or any thought,
word, or deed against the law of God." Deontology therefore
presupposes and is consequent upon Natural Theology. At the same time,
while Ethics indicate a valuable proof of the existence of God as the
requisite Object of Happiness, Deontology affords a proof of Him as
the requisite Lawgiver. Without God, man's rational desire is
frustrate, and man's conscience a misrepresentation of fact. [Footnote
1]

[Footnote 1: This is Cardinal Newman's proof of the existence of God
from Conscience: see pp. 124, 125, and _Grammar of Assent_, pp.
104-111, ed. 1895. With Newman's, "Conscience has both a critical and
a judicial office," compare Plato, _Politicus_, 260 B, [Greek:
sumpasaes taes gnostikaes to men epitaktikon meros, to de kritikon].
The "critical" office belongs to Ethics: the "judicial," or
"preceptive" office [Greek: to epitaktikon] to Deontology; and this
latter points to a Person who commands and judges, that is, to God.]

In this volume, pp. 1-108 make up the treatise on Ethics: pp. 109-176
that on Deontology.