Stereoscopy offers an excellent illustration to help us realize that space is an illusion created before our very eyes. If two photographs of the same object are taken from different angles, placed in a simple stereoscopic apparatus, and looked at through its little window, the resulting picture is no longer a flat two-dimensional thing but a bulky three-dimensional one. There has been added to the height and width of an ordinary photograph the new element of depth, which makes the object stand out in relief. What seems to be a tangible space has been created behind and in front of the object. The consequence is that the image is transformed in a startling manner from a lifeless representation to something that seems vividly real. When such an apparatus so obviously creates space for us we ought not to regard it as fantastic when mentalism tells us that the human mind subconsciously creates its own forms and projects them into a fancied space.
-- Notebooks Category 21: Mentalism > Chapter 2: The World As Mental > # 52