Figure 1 – Viewing the Kantian Illusion

Here is how you use the Kantian mirror projection. The print of Kant's head is affixed upside-down deep inside the box on the back of the wall closest to the observer. In figure 1 my assistant Joel looks through the round window at the top of the box and sees a ghostly image of Kant (right-side up now) projected by the mirror so that it hovers on his shoulders (the little cardboard stand on the platform outside the window.)

The camera obscura was in the 18th-century a metaphor for the "viewing-box of the mind." Real, fully 3D things in the world are projected on the screen in the mind as flat images.

Kant's box does the reverse. Here a flat image in the viewing box is projected out into the world as a spatial object. It was his metaphor for how we project spatial form onto the external things-in-themselves.