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Morality capable of Demonstration.
      §16. Upon this ground it is, that I am bold to think, that Moral-
ity is capable of Demonstration, as well as Mathematicks: Since the
precise real Essence of the Things moral Words stand for, may
be perfectly known; and so the Congruity, or Incongruity of the
Things themselves, be certainly discovered, in which consists
perfect Knowledge. Nor let any one object, that the names of
Substances are often to be made use of in Morality, as well as those
of Modes, from which will arise Obscurity. For as to Substances,
when concerned in moral Discourses, their divers Natures are not
so much enquir’d into, as supposed; v.g. when we say that Man is
subject to Law: We mean nothing by Man, but a corporeal rational
Creature: What the real Essence or other Qualities of that Creature
are in this Case, is no way considered. And therefore, whether a
Child or Changeling be a Man in a physical Sense, may amongst the
Naturalists be as disputable as it will, it concerns not at all the
moral Man, as I may call him, which is this immoveable unchange-
— 517 —
able Idea, a corporeal rational Being. For were there a Monkey, or any
other Creature to be found, that had the use of Reason, to such a
degree, as to be able to understand general Signs, and to deduce
Consequences about general Ideas, he would no doubt be subject to
Law, and, in that Sense, be a Man, how much soever he differ’d in
Shape from others of that Name. The Names of Substances, if they
be used in them, as they should, can no more disturb Moral, than
they do Mathematical Discourses: Where, if the Mathematicians
speak of a Cube or Globe of Gold, or any other Body, he has his clear
setled Idea, which varies not, though it may, by mistake, be applied
to a particular Body, to which it belongs not.
Locke Hum III, 11, §16, pp. 516-517