it is known that vision concerns what exists, not what does not exist. It is also known that vision is a purely positive reality, in which no element of nonexistence predominates. This is unlike tasting, which involves the transformation of some of what is tasted, or eating and drinking, which involve the transformation of food and drink and their entry into parts of the eater’s and drinker’s bodies. That can occur only through transformation and creation.

Since vision is a purely positive reality and concerns only what exists, the basis that makes it possible—and distinguishes what can be seen from what cannot—must either be wholly existent or involve an element of nonexistence. The latter is false, because nonexistence cannot affect what is purely existent. It therefore cannot be its cause, nor a condition or component of its cause, unless it entails something existent. How so? It is the existent element that acts upon existence, while that nonexistence serves as evidence for it, entails it, and so forth.

This becomes clear upon reflection. Those scholars who say that nonexistence can be the cause of something positive, a component of its cause, or a condition of its cause say this only in the context of inference from an indicator