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Ideas also of Substances must be conformable to Things.
      §24. But though Definitions will serve to explain the Names of
Substances, as they stand for our Ideas; yet they leave them not
without great imperfection, as they stand for Things. For our
Names of Substances being not put barely for our Ideas, but being
made use of ultimately to represent Things, and so are put in their
place, their signification must agree with the Truth of Things, as
well as with Men’s Ideas. And therefore in Substances, we are not
always to rest in the ordinary complex Idea, commonly received as
the signification of that Word, but must go a little farther, and
enquire into the Nature and Properties of the Things themselves,
and thereby perfect, as much as we can, our Ideas of their distinct
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Species; or else learn them from such as are used to that sort of
Things, and are experienced in them. For since ’tis intended their
Names should stand for such Collections of simple Ideas, as do really
exist in Things themselves, as well as for the complex Idea in other
Men’s Minds, which in their ordinary acceptation they stand for:
therefore to define their Names right, natural History is to be enquired
into; and their Properties are, with care and examination, to be
found out. For it is not enough, for the avoiding Inconveniencies in
Discourses and Arguings about natural Bodies and substantial
Things, to have learned, from the Propriety of the Language, the
common but confused, or very imperfect Idea, to which each Word
is applied, and to keep them to that Idea in our use of them: but we
must, by acquainting our selves with the History of that sort of
Things, rectify and settle our complex Idea, belonging to each
specifick Name; and in discourse with others, (if we find them
mistake us) we ought to tell, what the complex Idea is, that we make
such a Name stand for. This is the more necessary to be done by all
those, who search after Knowledge, and philosophical Verity, in
that Children being taught Words whilst they have but imperfect
Notions of Things, apply them at random, and without much
thinking, and seldom frame determined Ideas to be signified by
them. Which Custom, (it being easy, and serving well enough for
the ordinary Affairs of Life and Conversation) they are apt to
continue, when they are Men: And so begin at the wrong end,
learning Words first, and perfectly, but make the Notions, to which
they apply those Words afterwards, very overtly. By this means it
comes to pass, that Men speaking the proper Language of their
Country, i.e. according to Grammar-Rules of that Language, do yet
speak very improperly of Things themselves; and by their arguing
one with another, make but small progress in the discoveries of
useful Truths, and the Knowledge of Things, as they are to be
found in themselves, and not in our Imaginations; and it matters
not much, for the improvement of our Knowledge, how they are
call’d.
Locke Hum III, 11, §24, pp. 520-521