Once in possession of the ideas of a red rose and of the colour blue, we
may, in imagination, substitute blue for red; and thus obtain a complex
idea of a blue rose, which is not an actual copy of any complex
impression, though all its elements are such copies.
Hume has been criticised for making the distinction of impressions and
ideas to depend upon their relative strength or vivacity. Yet it would
be hard to point out any other character by which the things signified
can be distinguished. Any one who has paid attention to the curious
subject of what are called "subjective sensations" will be familiar with
examples of the extreme difficulty which sometimes attends the
discrimination of ideas of sensation from impressions of sensation, when
the ideas are very vivid, or the impressions are faint. Who has not
"fancied" he heard a noise; or has not explained inattention to a real
sound by saying, "I thought it was nothing but my fancy"? Even healthy
persons are much more liable to both visual and auditory spectra--that
is, ideas of vision and sound so vivid that they are taken for new
impressions--than is commonly supposed; and, in some diseased states,
ideas of sensible objects may assume all the vividness of reality.
If ideas are nothing but copies of impressions, arranged, either in the
same order as that of the impressions from which they are derived, or in
a different order, it follows that the ultimate analysis of the contents
of the mind turns upon that of the impressions.
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