the effect is the thing that counts, not the sensuous facts. (Wölfflin, Principles of Art History)

Giedion gave us a language for tackling the structural world of architecture and artifacts of many kinds in the ordinary environment. He learned this language from his preceptor, Wölfflin, whose Principles of Art History revolutionized the entire language of art criticism at the end of the nineteenth and the beginning of the twentieth centuries. (Stearn Interview)

McLuhan long knew of Heinrich Wölfflin (1864-1945) through Sigfried Giedion’s frequent mention of him as his mentor.  In Space, Time and Architecture, for example, which McLuhan read immediately after meeting Giedion in 1943 in St Louis, Giedion described his study with Wölfflin in explaining “where I come from“:

As an art historian, I am a disciple of Heinrich Wölfflin. In our personal contacts with him as well as through his distinguished lectures, we, his pupils, learned to grasp the spirit of an epoch. Wölfflin’s incisive analysis made clear to us the true meaning and significance of a painting or a piece of sculpture. He delighted in contrasting one period with another. He employed this method most effectively both in his teaching and in his books — in his Renaissance and Baroque [Renaissance und Barock] (1889), in Classical Art [Die klassische Kunst] (1899), in which the fifteenth century is opposed to the sixteenth, and even in his Principles of Art (Kunstgeschichtliche Grundbegriffe, 1915), which had just appeared when I studied under him at Munich. Many of his pupils have tried to emulate this method of contrasting styles, but none have achieved the same depth and directness. In my own first book (…)¹ I tried to follow Wölfflin’s method.

The great question was whether “the spirit of an epoch” might be defined rigorously via Grundbegriffe or fundamental principles such that investigation of it and of other epochs, alone and in combination, might be established on a new basis.

Now although McLuhan began reading Giedion in the mid-1940s and learning about Wölfflin through him, he seems not to have read Wölfflin himself until the late 1950s when art history began to play a decisive role in his thought. With Adolph Hildebrand, William Ivins and Ernst Gombrich, Wölfflin effected a revolution in McLuhan’s thinking at this time that provoked his notion of the elementary structure of experience and its resulting science or sciences. As cited below from The Gutenberg Galaxy, this was the notion that 

[there is] “unconscious inference” or mental action even in the most basic sense experience. “Tactility” or interplay among all the senses is² (…) the very mode of this “inference”. (81)

Like Hildebrand and Ivins, Wölfflin was treated by McLuhan chiefly in publications between 1960 and 1962:

Report on Project in Understanding New Media (1960)

• In 1915 Heinrich Wölfflin published his Principles of Art History which has since then revolutionized the study of many matters besides art. His entire approach confirms what I discovered about media: “the effect is the thing that counts, not the sensuous facts” (…) Switching attention to effects away from “the sensuous facts” highlighted (…) that the (…) visual (…) is in effect very tactile, resonant, and auditory. 
• But just how tricky the analysis of the quality of prints can be is revealed by Wölfflin (…) He analyses two engravings, one by Dürer, one by Rembrandt. The Dürer he shows has an SC [Subjective Completion] that is highly tactual. The Rembrandt has an SC that is highly visual. (…) The processing which the SI [Structural Impact or Sensory Impression] undergoes in each of us is bound to vary, just as the effect of radio or movie differs widely as it is processed through different cultures.

Inside the Five Sense Sensorium (1961)

• The art historian Heinrich Wölfflin taught the Hildebrand stress on visual forms as haptic or tangible-tactile — and Wölfflin’s pupil, Sigfried Giedion, embodied [!] it in his Space, Time and Architecture. 

The Gutenberg Galaxy (1962)

• 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)
• The relation of tactility to the visual, so necessary to an understanding of the fortunes of the phonetic alphabet, only became starkly defined after Cezanne. Thus Gombrich makes tactility a central theme of Art and Illusion, as does Heinrich Wölfflin in his Principles of Art History. And the reason for this new stress was that in an age of photography the divorce of the visual from the interplay of the other senses was pushed all the way into reaction. Gombrich records the stages of nineteenth-century discussion and analysis of “sense data” leading to the Helmholtz³ case for “unconscious inference” or mental action even in the most basic sense experience. “Tactility” or interplay among all the senses was felt to be the very mode of this “inference”… (81)