y have never
been put in action to produce a particular impression. We cannot form
to ourselves a just idea of the taste of a pine apple, without having
actually tasted it.

There is however one contradictory phaenomenon, which may prove, that it
is not absolutely impossible for ideas to go before their correspondent
impressions. I believe it will readily be allowed that the several
distinct ideas of colours, which enter by the eyes, or those of sounds,
which are conveyed by the hearing, 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, that each of them 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 well acquainted with colours of all kinds,
excepting 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, said will be sensible,
that there is a greater distance in that place betwixt the contiguous
colours, than in any other. Now I ask, whether it is 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 derived from the correspondent impressions; though
the instance is so particular and singular, that it is scarce worth
our observing, and does not merit that for it alone we should alter our
general maxim.

But besides this exception, it may not be amiss to remark on this head,
that the principle of the priority of impressions to ideas must be
understood with another limitation, viz., that as our ideas are images
of our impressions, so we can form secondary ideas, which are images of
the primary; as appears from this