Expression follows two paths of existence — exterior and interior. Of course in reality they are one, they are the same. We are taught that they are different. Interior existence cannot be “seen” with our senses, nor can others see them. Abstraction and the unconscious make up much of this terrain.
Photography captures the exterior existence. As a slice of life it is objective. It cannot see the interior on its own. It may be de-literalized through artistic manipulation.
Movements or schools of painting often are concerned almost totally with the abstract or the interior. Picasso often had little regard for the exterior. One can argue that color, shape and line has to do with the exterior, yet these components are used to express or represent the interior.
Emotion is also interior but also familiar in its expression in human behavior. We see the results of emotion. Behaviorists say that emotion does not exist since we cannot measure it.