nderstood. Of course,
when we are sitting in the picture palace we know that we see a flat
screen and that the object which we see has only two dimensions,
right-left, and up-down, but not the third dimension of depth, of
distance toward us or away from us. It is flat like a picture and never
plastic like a work of sculpture or architecture or like a stage. Yet
this is knowledge and not immediate impression. We have no right
whatever to say that the scenes which we see on the screen appear to us
as flat pictures.

We may become more strongly conscious of this difference between an
object of our knowledge and an object of our impression, if we remember
a well-known instrument, the stereoscope. The stereoscope, which was
quite familiar to the parlor of a former generation, consists of two
prisms through which the two eyes look toward two photographic views of
a landscape. But the two photographic views are not identical. The
landscape is taken from two different points of view, once from the
right and once from the left. As soon as these two views are put into
the stereoscope the right eye sees through the prism only the view from
the right, the left eye only the view from the left. We know very well
that only two flat pictures are before us; yet we cannot help seeing the
landscape in strongly plastic forms. The two different views are
combined in one presentation of the landscape in which the distant
objects appear much further away from us than the foreground. We feel
immediately the depth of things. It is as if we were looking at a small
plastic model of the landscape and in spite of our objective knowledge
cannot recognize the flat pictures in the solid forms which we perceive.
It cannot be otherwise, because whenever in practical life we see an
object, a vase on our table, as a solid body, we get the impression of
its plastic character first of all by seeing it with our two eyes from
two different points of view. The perspective in which our right eye
sees the things on our table is different from the perspective for the
left eye. Our plastic seeing therefore depends upon this combination of
two different perspective views, and whenever we offer to the two eyes
two such one-sided views, they must be combined into the impression of
the substantial thing. The stereoscope thus illustrates clearly that the
knowledge of the flat character of pictures by no means excludes the
actual perception of depth, and the question ar