For example: there be that say that the essence of body is EXTENSION; if it be so, we can never mistake in putting the essence of anything for the thing itself.

Let us then in discourse put extension for body, and when we would say that body moves, let us say that extension moves, and see how ill it will look.

He that should say that one extension by impulse moves another extension, would, by the bare expression, sufficiently show the absurdity of such a notion.

The essence of anything in respect of us, is the whole complex idea comprehended and marked by that name; and in substances, besides the several distinct simple ideas that make them up, the confused one of substance, or of an unknown support and cause of their union, is always a part: and therefore the essence of body is not bare extension, but an extended solid thing; and so to say, an extended solid thing moves, or impels another, is all one, and as intelligible, as to say, BODY moves or impels.

Likewise, to say that a rational animal is capable of conversation, is all one as to say a man; but no one will say that rationality is capable of conversation, because it makes not the whole essence to which we give the name man.
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