direction, the object more rapidly than ourselves; or, finally, from our
both moving in an opposite direction to this, ourselves more rapidly
than the object. There is thus always a variety of conceivable
explanations, and the action of past experience and association shows
itself very plainly in the determination of the direction of
interpretation. Thus, it is our instinctive tendency to take apparent
movement for real movement, except when the fact of our own movement is
clearly present to consciousness, as when we are walking, or when we are
sitting behind a horse whose movement we see. And so when the sense of
our own movement becomes indistinct, as in a railway carriage, we
naturally drift into the illusion that objects, such as trees, telegraph
posts, and so on, are moving, when they are perfectly still. Under the
same circumstances, we are apt to suppose that a train which is just
shooting ahead of us is moving slowly.

Similar uncertainties arise with respect to the relative movement of two
objects, the eye being supposed to be fixed in space. When two objects
seem to pass one another, it may be that they are both moving in
contrary directions, or that one only is moving, or finally, that both
are moving in the same direction, the one faster than the other.
Experience and habit here again suggest the interpretation which is most
easy, and not unfrequently produce illusion. Thus, when we watch clouds
scudding over the face of the moon, the latter seems moving rather than
the former, and the illusion only disappears when we fix the eye on the
moon and recognize that it is really stationary. The probable reason of
this is, as Wundt suggests, that experience has made it far easier for
us to think of small objects like the moon moving rapidly, than of large
masses like the clouds.[36]

The perception of distance, still more than that of direction, is liable