different colours, it must be no less so of the different shades of the
same colour; and each shade produces a distinct idea, independent of the
rest. For if this should be denied, it is possible, by the continual
gradation of shades, to run a colour insensibly into what is most remote
from it; and if you will not allow any of the means to be different, you
cannot, without absurdity, deny the extremes to be the same. Suppose,
therefore, a person to have enjoyed his sight for thirty years, and to
have become perfectly acquainted with colours of all kinds except one
particular shade of blue, for instance, which it never has been his
fortune to meet with. Let all the different shades of that colour,
except that single one, be placed before him, descending gradually from
the deepest to the lightest; it is plain that he will perceive a blank,
where that shade is wanting, and will be sensible that there is a
greater distance in that place between the contiguous colours than in
any other. Now I ask, whether it be possible for him, from his own
imagination, to supply this deficiency, and raise up to himself the idea
of that particular shade, though it had never been conveyed to him by
his senses? I believe there are few but will be of opinion that he can:
and this may serve as a proof that the simple ideas are not always, in
every instance, derived from the correspondent impressions; though this
instance is so singular, that it is scarcely worth our observing, and
does not merit that for it alone we should alter our general maxim.

17. Here, therefore, is a proposition, which not only seems, in itself,
simple and intelligible; but, if a proper use were made of it, might
render every dispute equally intelligible, and banish all that jargon,
which has so long taken possession of metaphysical reasonings, and drawn
disgrace upon them. All ideas, especially abstract ones, are naturally
faint and obscure: the mind has but a slender hold of them: they are apt
to be confounded with other resembling ideas; and when we have often