er, or
that it is along the same straight line as the other, but we cannot have
that immediate acquaintance with physical distances that we have with
distances in our private spaces, or with colours or sounds or other
sense-data. We can know all those things about physical space which a
man born blind might know through other people about the space of sight;
but the kind of things which a man born blind could never know about the
space of sight we also cannot know about physical space. We can know the
properties of the relations required to preserve the correspondence with
sense-data, but we cannot know the nature of the terms between which the
relations hold.

With regard to time, our _feeling_ of duration or of the lapse of time
is notoriously an unsafe guide as to the time that has elapsed by the
clock. Times when we are bored or suffering pain pass slowly, times when
we are agreeably occupied pass quickly, and times when we are sleeping
pass almost as if they did not exist. Thus, in so far as time is
constituted by duration, there is the same necessity for distinguishing
a public and a private time as there was in the case of space. But in so
far as time consists in an _order_ of before and after, there is no need
to make such a distinction; the time-order which events seem to have is,
so far as we can see, the same as the time-order which they do have. At
any rate no reason can be given for supposing that the two orders are
not the same. The same is usually true of space: if a regiment of men
are marching along a road, the shape of the regiment will look different
from different points of view, but the men will appear arranged in the
same order from all points of view. Hence we regard the order as true
also in physical space, whereas the shape is only supposed to correspond
to the physical space so far as is required for the preservation of the
order.

In saying that the time-order which events seem to have is the same as
the time-order which they really have, it is necessary to guard against
a possible misunderstanding. It must not be supposed that the various
states of different physical objects have the same time-order as the
sense-data which constitute the perceptions of those objects. Considered
as physical objects, the thunder and lightning are simultaneous; that is
to say, the lightning is simultaneous with the disturbance of the air in
the place where the disturbance begins, namely, where the lightning
is