But though these nominal essences of substances are made by the mind,
they are not yet made so arbitrarily as those of mixed modes. To the
making of any nominal essence, it is necessary, First, that the ideas
whereof it consists have such a union as to make but one idea, how
compounded soever. Secondly, that the particular ideas so united be
exactly the same, neither more nor less. For if two abstract complex
ideas differ either in number or sorts of their component parts, they
make two different, and not one and the same essence. In the first of
these, the mind, in making its complex ideas of substances, only follows
nature; and puts none together which are not supposed to have a union in
nature. Nobody joins the voice of a sheep with the shape of a horse;
nor the colour of lead with the weight and fixedness of gold, to be the
complex ideas of any real substances; unless he has a mind to fill his
head with chimeras, and his discourse with unintelligible words. Men
observing certain qualities always joined and existing together, therein
copied nature; and of ideas so united made their complex ones of
substances. For, though men may make what complex ideas they please, and
give what names to them they will; yet, if they will be understood WHEN
THEY SPEAK OF THINGS REALLY EXISTING, they must in some degree conform
their ideas to the things they would speak of; or else men's language
will be like that of Babel; and every man's words, being intelligible
only to himself, would no longer serve to conversation and the ordinary
affairs of life, if the ideas they stand for be not some way answering
the common appearances and agreement of substances as they really exist.


29. Our Nominal Essences of substances usually consist of a few obvious
qualities observed in things.

Secondly, Though the mind of man, in making its complex ideas of