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An Enquiry Concerning Human Understanding

16. There is, however, one contradictory phenomenon, which may prove that it is not
absolutely impossible for ideas to arise, independent of their correspondent impressions. I
believe it will readily be allowed, that the several distinct ideas of colour, which enter by
the eye, or those of sound, which are conveyed by the ear, are really different from each
other; though, at the same time, resembling. Now if this be true of 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 employed any
term, though without a distinct meaning, we are apt to imagine it has a determinate idea
annexed to it. On the contrary, all impressions, that is, all sensations, either outward or
inward, are strong and vivid: the limits between them are more exactly determined: nor is
it easy to fall into any error or mistake with regard to them. When we entertain, therefore,
any suspicion that a philosophical term is employed without any meaning or idea (as is
but too frequent), we need but enquire, from what impression is that supposed idea
derived? And if it be impossible to assign any, this will serve to confirm our suspicion.
By bringing ideas into so clear a light we may reasonably hope to remove all dispute,
which may arise, concerning their nature and reality.1