McLuhan wrote to Ezra Pound on December 21, 1948 about American ignorance of “the ideogram principle” aka of “the inclusive image“:

The American mind is not even close to being amenable to the ideogram principle as yet. The reason is simply this. America is 100% 18th Century. The 18th century had chucked out the principle of metaphor and analogy — the basic fact that as A is to B so is C to D. AB:CD. It can see AB relations. But relations in four terms are still verboten. This amounts to deep occultation of nearly all human thought for the U.S.A. I am trying to devise a way of stating this difficulty as it exists. Until stated and publicly recognized for what it is, poetry and the arts can’t exist in America. Mere exposure to the arts does nothing for a mentality which is incorrigibly dialectical. The vital tensions and nutritive action of ideogram remain inaccessible to this state of mind. (Letters 207)

Re the ” deep occultation of nearly all human thought”, see Dagwood and the ineradicable roots of our being (1944):

As excessive activity starved the other needs of man and sharpened the spirit of gain and commercialism, an unofficial blackout was ordained over the spiritual and intellectual areas of man’s nature.

Given this “occultation” or “blackout”, art, religion and philosophy — “the spiritual and intellectual areas of man’s nature” — were now surface phenomena serving only for manipulation:

The press, the pulps, the slicks, and Hollywood — it is a great nursery world of sensations, thrills, and wide-eyed child-like myopia. (Dagwood)

The result was that Americans aka the modern world

can never discover nourishment for these roots in popular art and literature.