— 455 —
But not so arbitrary as mixed Modes.       §28. 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 an 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 an union in Nature. No body 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
— 456 —
with Chimaeras, and his Discourse with unintelligible Words. Men,
observing certain Qualities always join’d 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.
Locke Hum III, 6, §28, pp. 455-456