that all sensation being produced in us only by different degrees and modes of motion in our animal spirits, variously agitated by external objects, the abatement of any former motion must as necessarily produce a new sensation as the variation or increase of it; and so introduce a new idea, which depends only on a different motion of the animal spirits in that organ.
5.

Negative names need not be meaningless.
But whether this be so or not I will not here determine, but appeal to every one's own experience, whether the shadow of a man, though it consists of nothing but the absence of light (and the more the absence of light is, the more discernible is the shadow) does not, when a man looks on it, cause as clear and positive idea in his mind, as a man himself, though covered over with clear sunshine?
And the picture of a shadow is a positive thing.

Indeed, we have negative names, to which there be no positive ideas; but they consist wholly in negation of some certain ideas, as SILENCE, INVISIBLE; but these signify not any ideas in the mind but their absence.
6.

Whether any ideas are due to causes really private.
And thus one may truly be said to see darkness.

For, supposing a hole perfectly dark, from whence no light is reflected, it is certain one may see the figure of it, or it may be painted; or whether the ink I write with makes any other idea, is a question.