McLuhan first read The Problem of Form by Adolf von Hildebrand (1847-1921) in the late 1950s when, not coincidentally, he made his great breakthrough into the scientific investigation of experience in a ‘classroom without walls’.  Along with other art historians and theorists like Heinrich Wölfflin (1864-1945), William Ivins (1881-1961) and Ernst Gombrich (1909-2001), all of whom McLuhan initially encountered in these same years, Hildebrand helped McLuhan perceive how a number of seemingly disparate lines of inquiry might suddenly crystallize into a single coherent structure:

• Aristotle and Thomas on the fundamental role of sensibility in human experience and thought
• Eliot on the “dissociation of sensibility”
• Mallarmé, Eliot, and Joyce on impersonality and the poetic genesis of experience
• Innis and Havelock on media as determinants of experience
• Innis, Havelock and Richards on media as forms of sensibility

McLuhan treated Hildebrand primarily in three publications in 1961 and 1962:

1961 — ‘Inside the Five Sense Sensorium’

• in 1893 Adolf Hildebrand the sculptor published a small book called The Problem of Form. He insisted that true vision must be much imbued with tangibility, and that creative, aesthetic awareness was touching and making. Such was the timeliness of his insistence that the theme of artistic vision as tangible, tactile, and based on the interplay of the sense[s] began to enjoy acceptance in poetry and painting alike. The art historian Heinrich Wölfflin taught the Hildebrand stress on visual forms as haptic or tangible-tactile — and his pupil Sigfried Giedion embodied it in his Space, Time and Architecture.
• television offers a massive Bauhaus program of the re-education for North American sense life. That is (…) the television image is, in effect, a haptic, tactile, or synesthetic mode of interplay among the senses, a fulfillment on a popular plane of the aesthetic program of Hildebrand, Berenson, Wölfflin, Paul Klee, and Giedion.
• television has the power of imposing its own conventions and assumptions on the sensibilities of the viewer. It has the power of translating the Western literate back into the world of non-literate synesthesia, just as effectively as the phonetic alphabet can hoick the native out of his haptic matrix into a world of mechanistic individualism, and sequential cause-and-effect relations. Far from regarding these developments with any feeling of euphoria, I would suggest that when Hildebrand conducted his campaign for tactility against mere retinal pictorial impression, he was in the centre of a great cultural current which  from Cezanne in painting to Conrad in literature, swept up all into the ‘Heart of Darkness’ or ‘the Africa within’. This drive towards ‘spontaneity of consciousness’ fostered the child cult, as well as primitivism of many varieties, but it represented a rebellion against merely visual culture — a rebellion that had begun in the eighteenth century with Rousseau and others.

1962 — addition to ‘Joyce, Aquinas, and the Poetic Process’ (originally published in 1951)

• One reason why Hildebrand had such an immediate effect on the artists of his time is that he was able to explain why synesthesia is [the elementary structure of]¹ human experience, [and]² he showed why the [relative] isolation of the retinal or [of] the haptic or [of] any [one sort of sense] impression was, artistically, a disaster. Humanly speaking, the [imbalance]³ of the senses is the formula for insanity. (…) Hildebrand saw with absolute clarity that photography and photo-engraving were effecting [a relative]⁴ isolation of retinal impression from the other senses. After his book appeared, the critics like Bernard Berenson and later Roger Fry and Clive Bell began to stress the urgency of haptic, tactile [balance with]⁵ retinal impression.(…) The role of Hildebrand in shaping the vortex idea of Lewis, Pound, and TS Eliot is as decisive as his effect on Heinrich Wölfflin and the Bauhaus.
• Always with Hildebrand, then, is the prime stress on the interplay of [gen obj!] knowing and making. The intelligible is Being, says Aquinas; and it is the splitting up of knowing and making which impoverishes art, experience, and Being alike says Hildebrand in a passage relevant to “dissociation of sensibility”. 
• Hildebrand points the corollary for Art: “If one would speak, then, of a mission of Art, it can be no other than this: in spite of all temporal eccentricities, to reestablish and make felt the sound and natural relations between our thought and sense activities.” [123] This is surely close to Baudelaire’s notion that the role of Art is to diminish the traces of original sin. 
• [Hildebrand ] rejects the “innocent eye” notion of art as postulating a separation rather than an interplay of the senses: “The height of positivism would be attained if we could perceive things with the inexperience of a new-born child. This theory would lead us to regard the sculptor’s art as appealing exclusively to the tactual-kinesthetic sense of the esthetic percipient; the painter’s art, on the other hand, as appealing entirely to the visual sense quite apart from all experience of form. (…) In true Art the actual form has its reality only as an effect. By conceiving Nature as a relation of kinesthetic ideas to visual impressions, all combined and interrelated in a totality, [and this as [underlying] cause of the effect Art presents], Art frees her [Nature] of change and chance.”

1962 — The Gutenberg Galaxy

• if a new technology extends one or more of our senses outside us into the social world, then new ratios among all of our senses will occur in that particular culture. It is comparable to what happens when a new note is added to a melody. And when the sense ratios alter in any culture then what had appeared lucid before may suddenly be opaque, and what had been vague or opaque will become translucent. As Heinrich Wölfflin stated the matter in 1915, in his revolutionary Principles of Art History (p. 62) “the effect is the thing that counts, not the sensuous facts.” Wölfflin began working from the discoveries of the sculptor Adolf von Hildebrand, whose Problem of Form in the Figurative Arts had first clearly explained the disorder in ordinary human sense perception, and the role of art in clarifying this confusion. Hildebrand had shown how tactility was a kind of synesthesia or interplay among the senses, and as such, was the core of the richest art effects. (41)
• [McLuhan citing Ernst Gombrich at length from Art and Illusion:] “Two German thinkers are prominent in this story. One is the critic Konrad Fiedler, who insisted, in opposition to the impressionists, that “even the simplest sense impression that looks like merely the raw material for the operations of the mind is already a mental fact, and what we call the external world is really the result of a complex psychological process.” But it was Fiedler’s friend, the neoclassical sculptor Adolf von Hildebrand, who set out to analyze this process in a little book called The Problem of Form in the Figurative Arts, which came out in 1893 and gained the ear of a whole generation. Hildebrand, too, challenged the ideals of scientific naturalism by an appeal to the psychology of perception: if we attempt to analyze our mental images to discover their primary constituents, we will find them composed of sense data derived from vision and from memories of touch and movement. A sphere, for instance, appears to the eye as a flat disk; it is touch which informs us of the properties of space and form. Any attempt on the part of the artist to eliminate this knowledge is futile, for without it he would not perceive the world at all. His task is, on the contrary, to compensate for the absence of movement in his work by clarifying his image and thus conveying not only visual sensations but also those memories of touch which enable us to reconstitute the three-dimensional form in our minds. It is hardly an accident that the period when these ideas were so eagerly debated was also the period when the history of art emancipated itself from antiquarianism, biography, and aesthetics. Issues which had been taken for granted so long suddenly looked problematic and required reassessment. When Bernard Berenson wrote his brilliant essay on the Florentine painters, which came out in 1896, he formulated his aesthetic creed in terms of Hildebrand’s analysis. With his gift for the pregnant phrase, he summed up almost the whole of the sculptor’s somewhat turgid book in the sentence: ‘The painter can accomplish his task only by giving tactile value to retinal impressions’.” (82)