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 are situated in our private spaces, either the space
of sight or the space of touch or such vaguer spaces as other senses
may give us. If, as science and common sense assume, there is one public
all-embracing physical space in which physical objects are, the relative
positions of physical objects in physical space must more or less
correspond to the relative positions of sense-data in our private
spaces. There is no difficulty in supposing this to be the case. If we
see on a road one house nearer to us than another, our other senses will
bear out the view that it is nearer; for example, it will be reached
sooner if we walk along the road. Other people will agree that the house
which looks nearer to us is nearer; the ordnance map will take the
same view; and thus everything points to a spatial relation between the
houses corresponding to the relation between the sense-data which we see
when we look at the houses. Thus we may assume that there is a physical
space in which physical objects have spatial relations corresponding to
those which the corresponding sense-data have in our private spaces. It
is this physical space which is dealt with in geometry and assumed in
physics and astronomy.

Assuming that there is physical space, and that it does thus correspond
to private spaces, what can we know about it? We can know _only_ what is
required in order to secure the correspondence. That is to say, we can
know nothing of what it is like in itself, but we can know the sort
of arrangement of physical objects which results from their spatial
relations. We can know, for example, that the earth and moon and sun
are in one straight line during an eclipse, though we cannot know what
a physical straight line is in itself, as we know the look of a straight
line in our visual space. Thus we come to know much more about the
_relations_ of distances in physical space than about the distances
themselves; we may know that one distance is greater than anoth