Among the various intellectual disciplines and the emotions, another characteristic difference is that of their proofs or verification. The most gross form of proof is that of physical tangibility, and the most subtle or delicate, that of (emotional) feeling.

The existence of a physical body is proven by its occupation of its own impenetrable space. Where a wall stands no object can pass. Its tangible and impenetrable presence testifies to its nature and existence.

Some forces act on physical bodies but lack the spatial tenancy of the physical, for example, the faculty of vision. Vision acts upon physical, space-occupying objects, and vision verifies their existence. In a flash, the scene passes into the realm of memory, and the vision-space is occupied by another sight. In turn, this one will be displaced by yet a third.

At the time of viewing, each scene occupies vision-space no less impenetrable than physical tenancy. One cannot concentrate vision on two scenes simultaneously. All the senses and powers of man are concentrated during the viewing and are affected accordingly: pleasure may ensue, or anguish. At any rate, the scene occupies “space” in all the powers of the soul, and with its disappearance another scene occupies that same space.

When one wishes to recall a scene, the scene, with all its details and the accompanying emotions aroused by it, may reappear. This indicates that the scenes are conserved in the reservoir of the memory and occupy space, comparable in its own terms to physical space. But this space is not impenetrable, since many scenes are conserved concurrently, and all may be readily recalled.

Thus, some forces act upon physical bodies, but can tolerate, without effect, coincident tenancy with other forces that act upon similar physical bodies.