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Instance in Gold.
      §8. All Gold is fixed, is a Proposition whose Truth we cannot be
certain of, how universally soever it be believed. For if, according to
the useless Imagination of the Schools, any one supposes the term
Gold to stand for a Species of Things set out by Nature, by a real
Essence belonging to it, ’tis evident he knows not what particular
Substances are of that Species; and so cannot, with certainty,
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affirm any thing universally of Gold. But if he makes Gold stand for a
Species, determined by its nominal Essence, let the nominal Essence,
for example, be the complex Idea of a Body, of a certain yellow colour,
malleable, fusible, and heavier than any other known; in this proper
use of the word Gold, there is no difficulty to know what is, or is not
Gold. But yet no other Quality can with certainty be universally
affirmed or denied of Gold, but what hath a discoverable connexion,
or inconsistency with that nominal Essence. Fixedness, for example,
having no necessary connexion, that we can discover, with the
Colour, Weight, or any other simple Idea of our complex one, or with
the whole Combination together; it is impossible that we should
Locke Hum IV, 6, §8, pp. 582-583