Locke’s Theory of Perception 239
in the traditional five ways of sensing, together with the
status of the qualities that are sensed.
Locke names the subject-matter of his Essay—a part of
which is sensation—the “Science of Signs,”1 because all per-
ception makes use of various elements which signify others
beyond them. Even colors and odors “mean” the presence
of certain kinds of bodies in the external world. Hence it is
clear that such a science, being in part a description of how
mind communicates with matter, must overlap the science of
mind on the one hand, and the science of material existence
on the other. I shall begin with Locke’s view of physical ex-
istence, then straddle the borderland between matter and
mind, and, from the vantage-point of this great divide, sur-
vey briefly the realm of mind. First, however, I shall per-
form an experiment to create in us a keen sense for the prob-
lem of perception, and for the apparent inevitability of
Locke’s general conclusions in this regard.
Imagine here before us a small white marble attached to
the end of a slender steel rod. Now suppose by mechanical
means we cause the end of the rod that bears the white ob-
ject to oscillate rapidly between two points about six inches
apart. What will you then see? If your line of vision is at
right angles to the rod’s plane of oscillation, you will see a
curved, white, motionless line about six inches long. That is,
you will see with your eyes a color apparently qualifying a
region of space that it does not really qualify. Thus the ar-
gument collapses that colors must surely be where they seem
to be from the standpoint of animal perception. But consider
now the white marble as itself a stationary object lying in the
palm of your hand. Is it not a solid object, its whiteness
really existing in the region of space occupied by the marble?
The answer of physics is a curt negative. As Russell says,
ɪ Essay, ɪv, 21, 4.