fire, as qualities inherent in it, the
changes in form and color which it produces in wax and the pain which it
causes in the finger brought into proximity with it. The warmth and the
brightness of the blaze, the redness, the pleasant taste, and the aromatic
odor of the strawberry, exist in these bodies merely as the power to
produce such sensations in us by stimulation of the skin, the eye, the
palate, and the nose. If we remove the perceptions of them, they disappear
as such, and their causes alone remain--the bulk, figure, number, texture,
and motion of the insensible particles. The ground of the illusion lies in
the fact that such qualities as color, etc., bear no resemblance to their
causes, in no wise point to these, and in themselves contain naught of
bulk, density, figure, and motion, and that our senses are too weak
to discover the material particles and their primary qualities.--The
distinction between qualities of the first and second order--first advanced
by the ancient atomists, revived by Galileo and Descartes on the threshold
of the modern period, retained by Locke, and still customary in the natural
science of the day--forms an important link in the transition from the
popular view of all sense-qualities as properties of things in themselves
to Kant's position, that spatial and temporal qualities also belong
to phenomena alone, and are based merely on man's subjective mode of
apprehension, while the real properties of things in themselves are
unknowable.

Thus far the procedure of the understanding has been purely passive. But
besides the capacity for passively receiving simple ideas, it possesses the
further power of variously combining and extending these original ideas
which have come into it from without, of working over the material given
in sensation by the combination, relation, and separation of its various
elements. In this it is active, but not creative. It is not able to form
new simple ideas (and just as little to destroy such as already exist), but
only freely to combine the elements furnished without its assistance by
perception (or, following the figure mentioned above, to combine into
syllables and words the separate letters of sensation). Complex ideas arise
from simple ideas through voluntary combination of the latter.

Perception is the first step toward knowledge. After perception the most
indispensable faculty is retention, the prolonged consciousness of present
ideas and the reviva