of its parts."--(I. p. 331).

But, leaving the question of the proper definition of mind open for the
present, it is further a matter of direct observation, that, when we
take a general survey of all our perceptions or states of consciousness,
they naturally fall into sundry groups or classes. Of these classes, two
are distinguished by Hume as of primary importance. All "perceptions,"
he says, are either "_Impressions_" or "_Ideas_."

Under "impressions" he includes "all our more lively perceptions, when
we hear, see, feel, love, or will;" in other words, "all our sensations,
passions, and emotions, as they make their first appearance in the soul"
(I. p. 15).

"Ideas," on the other hand, are the faint images of impressions in
thinking and reasoning, or of antecedent ideas.

Both impressions and ideas may be either _simple_, when they are
incapable of further analysis, or _complex_, when they may be resolved
into simpler constituents. All simple ideas are exact copies of
impressions; but, in complex ideas, the arrangement of simple
constituents may be different from that of the impressions of which
those simple ideas are copies.

Thus the colours red and blue and the odour of a rose, are simple
impressions; while the ideas of blue, of red, and of rose-odour are
simple copies of these impressions. But a red rose gives us a complex
impression, capable of resolution into the simple impressions of red
colour, rose-scent, and numerous others; and we may have a complex idea,
which is an accurate, though faint, copy of this complex impression.
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
s