28. But not so arbitrary as mixed modes.
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.