Knowledge, life, courage, patience and ignorance are not perceived as colors, sounds or tactile qualities. We understand them conceptually. We may see a courageous person acting bravely, but courage itself is not a visible pigment or physical outline.
Thus “knowledge is light” compares a conceptual term, knowledge, with a perceptible model, light. The fact that one term is sensory does not make both sensory.
This distinction prevents two common errors: identifying the terms before considering their mode of apprehension, and assuming that the nature of a term automatically determines the nature of the shared point of resemblance. These must be assessed separately.