Locke’s Theory of Perception 255
something that a moving particle could bump into, how could
the said motion cause anything to happen to mind?1 The
only way an atom can cause anything is by collision, Locke
himself has told us. And since the contact of matter with
mind occurs in the region of the sense-organ, the sense-organ
might be described either as matter blossoming forth into
mind or as mind congealing into matter, to make causation
between them possible. Now let us examine this “dense”
region of the mind “immersed in flesh” as it is being stimu-
lated by say the organ of sight. In this mental region a
colored patch appears as the effect of the physical stimula-
tion, and this is what Locke calls a “simple” sensory idea,
or idea of sensation. Likewise, there are simple sensory
ideas resulting in the mind from touch or smell or taste or
hearing. Locke attempts no further definition of these simple
sensory units, on the ground that what is so simple cannot
be defined. Only what is complex is susceptible of definition.
Let him who would have acquaintance with the simple ideas
of sense open his eyes and unstop his ears. If he is blind or
deaf, he cannot know colors or sounds.
Now the ideas of sense are perceived to have certain quali-
ties, and this at last brings us to the means of escape from
the realm of mind into the realm of matter. Some of these
qualities resemble the qualities of real physical objects, such
that by knowing them in the mind, we shall, indirectly, obtain
knowledge of the properties of bodies. For example, the
properties of being extended and moving in space, of being
solid and numerable, are exhibited by the tactual and visual
sensations. In having direct acquaintance with these proper-
ties in the mind, we have indirect knowledge, according to
Locke, of the nature of physical objects, since such properties
ɪ Strictly speaking, a moving particle does not “bump into” mind ; when
the particles of a sense organ move, a sensation is “annexed” to the motion.