very much farther. In agreement with Berkeley's
ultra-nominalism, which combats even the possibility of abstract ideas, he
yet does not follow him to the extent of denying external reality. On the
other hand, he carries out more consistently Berkeley's hint that immediate
sensation includes less than is ascribed to it (_e.g._, that by vision
we perceive colors only, and not distance, etc.), as well as his
principle--destructive to the certainty of our knowledge of nature--that
there is no causality among phenomena; and brings the question of substance
to, the negative conclusion, that there is no need whatever for a support
for groups of qualities, and, therefore, that substantiality is to be
denied to immaterial as well as to material beings. The points in Locke's
philosophy which seemed to Hume to need completion were different from
those at which Berkeley had struck in. The antithesis of rational and
empirical knowledge is more sharply conceived; the combination of ideas is
not left to the choice of the understanding but placed under the dominion
of psychological laws; and to the distinction between outer and inner
experience (to the former of which priority is conceded, on the ground that
we must have had an external sensation before we can, through reflection,
be conscious of it as an internal phenomenon), there is added a second, as
important as the other and crossing it, between impressions and ideas, of
which the former are likewise made prior to the latter.

Everyone will acknowledge the considerable difference between a sensation
actually present (of heat, for instance) and the mere idea of one
previously experienced, or shortly to come. This consists in the greater
force, liveliness, and vividness of the former. Although these two classes
of states (the idea of a landscape described by a poet and the perception
of a real one, anger and the thought of anger) are only quantitatively
distinct, they are scarcely ever in danger of being confused--the most
lively idea is always less so than the weakest perception. The actual,
outer or inner, sensations may be termed impressions; the weaker images of
memory or imagination, which they leave behind them, ideas. Since nothing
can gain entrance to the soul except through the two portals of outer and
inner experience, there is no idea which has not arisen from an impression
or several such; every idea is the image and copy of an impression. But
as the understanding and ima