Despotism of the Eye
The phrase is Coleridge's. Both he and Barfield mean by it the literalizing tendency--a prime contributor to modern idolatry--to accept as real only that which can be empirically, "objectively" perceived with the senses. Such an outlook necessarily relegates the realm of spirit to the nonexistent.

The despotism of the eye, Barfield notes, is well summarized in the scholastic maxim "De non Apparentibus non existentibus eadem est ratio" (the nonphenomenal is the nonexistent) (RM 117).

See in particular What Coleridge Thought, passim.