The Problems of Philosophy | Page 9

man of science, and in no way assist him in explaining the
phenomena.
It is sometimes said that 'light is a form of wave-motion', but this is misleading, for the
light which we immediately see, which we know directly by means of our senses, is not a
form of wave-motion, but something quite different--something which we all know if we
are not blind, though we cannot describe it so as to convey our knowledge to a man who
is blind. A wave-motion, on the contrary, could quite well be described to a blind man,
since he can acquire a knowledge of space by the sense of touch; and he can experience a
wave-motion by a sea voyage almost as well as we can. But this, which a blind man can
understand, is not what we mean by _light_: we mean by light just that which a blind man
can never understand, and which we can never describe to him.
Now this something, which all of us who are not blind know, is not, according to science,
really to be found in the outer world: it is something caused by the action of certain
waves upon the eyes and nerves and brain of the person who sees the light. When it is
said that light is waves, what is really meant is that waves are the physical cause of our
sensations of light. But light itself, the thing which seeing people experience and blind
people do not, is not supposed by science to form any part of the world that is
independent of us and our senses. And very similar remarks would apply to other kinds of
sensations.
It is not only colours and sounds and so on that are absent from the scientific world of
matter, but also space as we get it through sight or touch. It is essential to science that its
matter should be in a space, but the space in which it is cannot be exactly the space we
see or feel. To begin with, space as we see it is not the same as space as we get it by the
sense of touch; it is only by experience in infancy that we learn how to touch things we
see, or how to get a sight of things which we feel touching us. But the space of science is
neutral as between touch and sight; thus it cannot be either the space of touch or the space
of sight.
Again, different people see the same object as of different shapes, according to their point
of view. A circular coin, for example, though we should always judge it to be circular,
will look oval unless we are straight in front of it. When we judge that it is circular, we
are judging that it has a real shape which is not its apparent shape, but belongs to it
intrinsically apart from its appearance. But this real shape, which is what concerns
science, must be in a real space, not the same as anybody's apparent space. The real
space is public, the apparent space is private to the percipient. In different people's

private spaces the same object seems to have different shapes; thus the real space, in
which it has its real shape, must be different from the private spaces. The space of science,
therefore, though connected with the spaces we see and feel, is not identical with them,
and the manner of its connexion requires investigation.
We agreed provisionally that physical objects cannot be quite like our sense-data, but
may be regarded as causing our sensations. These physical objects are in the space of
science, which we may call 'physical' space. It is important to notice that, if our
sensations are to be caused by physical objects, there must be a physical space containing
these objects and our sense-organs and nerves and brain. We get a sensation of touch
from an object when we are in contact with it; that is to say, when some part of our body
occupies a place in physical space quite close to the space occupied by the object. We see
an object (roughly speaking) when no opaque body is between the object and our eyes in
physical space. Similarly, we only hear or smell or taste an object when we are
sufficiently near to it, or when it touches the tongue, or has some suitable position in
physical space relatively to our body. We cannot begin to state what different sensations
we shall derive from a given object under different circumstances unless we regard the
object and our body as both in one physical space, for it is mainly the relative positions of
the object and our body that determine what sensations we shall derive from the object.
Now our sense-data