Locke’s Theory of Perception 251
especially since on the same page he goes on to write that the
motions effected in the retina are continued to the brain and
that this chain of intra-organic motions produces sensations
in our minds. Thus Locke remains unscathed by the demon-
strations of modern experimental psychology which prove,
by the relative sizes of retinal images, that the retinal image
is itself not seen, but rather the shape and size of something
else.
Now why is it that when the above-mentioned particles
rebounding from the marble strike the eye, we see white
rather than green or blue? The color that is actually pro-
duced in any given circumstance depends primarily, accord-
ing to Locke, on the joint rotational and progressive motions
of the particles of light. That is, the velocity of approach
to the eye and the rate of rotation of the particle determine
what particular color is to occur, and that color becomes
more intense as the number of such particles, moving thus,
increases. To all this let it be added in fairness to Locke,
that the color white is not to be simply identified with certain
kinds of motion and texture of parts. Colors stand to such
motions and physical textures only as mental effects to ma-
terial causes. And Locke does not even pretend to have
detailed knowledge of the way in which the material causes
produce color-sensations. One thing however he is sure
about, and that is that sensations are caused in the mind by
the impact of material particles against the organism, what-
ever their motions or ultimate natures may be. This is
called the “causal” theory of perception, and in itself is
reasonable enough both to common and to scientific sense,
though certain of its consequences—which we are about to
trace—are not nearly so congenial.
So far we have limited our discussion to the class of visual
sensations. This was not at all necessary to drive home