ct back ether waves of
a rate of 450 billions a second: "white" objects reflect all rates;
"black" objects reflect none.

The case is no different with regard to sound. When we speak of a sound
coming from a bell, what we really mean is that the vibrations of the
bell have set up waves in the air between it and our ear, which have
produced corresponding vibrations in the ear; that a nerve current was
thereby produced; and that a sound was heard. But the sound (i.e.,
sensation) is a mental thing, and exists only in our own consciousness.
What passed between the sounding object and ourselves was waves in the
intervening air, ready to be translated through the machinery of nerves
and brain into the beautiful tones and melodies and harmonies of the
mind. And so with all other sensations.

THE THREE SETS OF FACTORS.--What exists outside of us therefore is a
_stimulus_, some form of physical energy, of a kind suitable to excite
to activity a certain end-organ of taste, or touch, or smell, or sight,
or hearing; what exists within us is the _nervous machinery_ capable of
converting this stimulus into a nerve current which shall produce an
activity in the cortex of the brain; what results is the _mental object_
which we call a _sensation_ of taste, smell, touch, sight, or hearing.

2. THE NATURE OF SENSATION

SENSATION GIVES US OUR WORLD OF QUALITIES.--In actual experience
sensations are never known apart from the objects to which they belong.
This is to say that when we see _yellow_ or _red_ it is always in
connection with some surface, or object; when we taste _sour_, this
quality belongs to some substance, and so on with all the senses. Yet by
sensation we mean only _the simple qualities of objects known in
consciousness as the result of appropriate stimuli applied to
end-organs_. We shall later see how by perception these qualities fuse
or combine to form objects, but in the present chapter we shall be
concerned with the qualities only. Sensations are, then, the simplest
and most elementary knowledge we may get from the physical world,--the
red, the blue, the bitter, the cold, the fragrant, and whatever other
qualities may belong to the external world. We shall not for the present
be concerned with the objects or sources from which the qualities may
come.

To quote James on the meaning of sensation: "All we can say on this
point is that _what we mean by sensations are first things in the way of
consciousness_. They a