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 to be illusory.