In the first half of 1946, or perhaps already in 1945, McLuhan wrote a short essay titled ‘Time, Life, and Fortune’.¹ The following passage from the essay has a number of remarkable anticipations of ideas McLuhan would develop over the next 15 years: 

Time, Life, and Fortune represent three levels of irresponsible politics in much the same sense as Hollywood is willy-nilly a political force. That is, neither T.L.F. nor Hollywood attempts to hold up any kind of object or program for detached observation or appraisal. But both arrange their exhibits in suchwise as to manipulate the standardized reflexes of a semi-hypnotized and mentally helpless audience. So that the art of the movie is not to be judged by the invention and arrangement of images but by the effect which those images have on the contents of the mind or guts of a spectator. Likewise with T.L.F. It isn’t the worth or character of the image or statement presented which is of any political significance but the effect [on its “semi-hypnotized and mentally helpless audience”] which it is observed to have [by]² a [“detached” yet] sharply focussed reader [who would thereby be capable of “appraisal”]. Needless to say, the [“semi-hypnotized and mentally helpless”] reader is not the one to do the focussing. He is held in position.

McLuhan first used the phrase “the medium is the message” in print in 1958³ (although Carl Williams improbably recalls it already from 1953-1954)⁴. However that may be, as seen in the passage from his ‘Time, Life, and Fortune’ article cited above, the basic idea was already present to him 10 or 15 years before:

the art of the movie is not to be judged by the invention and arrangement of images but by the effect which those images have on the (…) mind or guts of a spectator. Likewise with T.L.F. It isn’t the worth or character of the image or statement presented which is of any political significance but the effect…

What the movie and the magazine were, namely media, had not yet come into focus at this time. This would soon come through McLuhan’s exposure at UT to the work of Harold Innis and of Eric Havelock and, beginning around the same time, his study of Stéphane Mallarmé. But it was already plain to McLuhan what these forms of entertainment and instruction were not, namely, they were not “image or statement” —  that is, they were not content or message.

Further, while the phrase, “the medium is the massage“, would first appear more than twenty years later, that notion, too, is already clear here:

both [T.L.F. and Hollywood] arrange their exhibits in suchwise as to manipulate the (…)  audience. So that the art of the movie is not to be judged by the invention and arrangement of images but by the effect which those images have (…). Likewise with T.L.F. It isn’t the worth or character of the image or statement presented which is of any political significance but the effect…

Further still, as McLuhan recorded in Report on the Project in Understanding New Media:

Early in 1960 it dawned on me that the sensory impression proffered by a medium like movie or radio, was not the sensory effect obtained.⁵ Radio, for example, has an intense visual effect on listeners. But then there is the telephone which also proffers an auditory impression, but has no visual effect. In the same way television is watched but has a very different effect from movies. These observations led to a series of studies of the media, and to the discovery of basic laws concerning the sensory effects of various media. These will be found in this report.  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“…⁶

Moving beyond focus on media and their massaging effects, the great leap made by McLuhan here in January 1960⁷ (however inchoate it may have been at the time and, indeed, may largely have remained for McLuhan despite another twenty years of probing it) was the notion that what a medium is — is “the sensory effect obtained” (“outputs”) and not “the sensory impression proffered” (inputs).⁸  Hence, in one of the examples given by him in this passage, while “the sensory impression proffered” by radio is plainly auditory, “the sensory effect obtained” is at least partly “visual” or even “intensely” so. Moreover, it is this “sensory effect obtained” that accounts for “the penetrative powers” (a notion borrowed by McLuhan from Innis⁹) of media. Hence McLuhan began his 1953 memorial essay on Innis, who died at the end of 1952, as follows:

Often misunderstood or ignored by those who had admired his classic study of the Fur Trade, the work of the later Innis was a shift in attention from the trade-routes of the external world to the trade routes of the mind. Technology, he saw, had solved the problem of production of commodities and had already turned to the packaging of information. And the penetrative powers of the pricing system were as nothing beside the power of the new media of communication to penetrate and transform all existing institutions and patterns of thought. (‘The Later Innis’, Queen’s Quarterly, 60:3, 1953, 385-394)

The key to this insight into the being and power of media was simultaneity or synchronicity. Electric technology introduced “allatonceness” where cause and effect were not sequential but instantaneous. So the effect of a medium (subj gen) could now be seen as the cause of that medium (subj gen) since the two were interlocked not in chronological if-then fashion, but in simultaneous time.  The very being of a medium was the translation or metamorphosis it effected into an altered psychological and physical environment. As such, this was not an effect or outcome which a medium could fail to impose!  Rather, exactly this psycho-physical-environmental im-position was what a medium was! As McLuhan remarked in Understanding Media:

extension [of any sense] also demands new ratios or new equilibriums among the other [sense] organs and [previous] extensions of the body. There is, for example, no way of refusing to comply with the new sense ratios or sense “closure” evoked by the TV image. (45)

Now all this from the 1950s, culminating in his breakthrough writings in 1960-1964, was very close to what McLuhan could already see in 1946!

the art of the movie is not to be judged by the invention and arrangement of images but by the effect which those images have on the contents of the mind or guts of a spectator. (…) He is held in position.

Media, he would increasingly come to see, were position binders (another Innis notion) or variable time-space matrices. It followed that a general relativity theory covering the range of their possible forms was required which could therefore be called — Understanding Media.

This is the cover of McLuhan’s 1960 Report on the Project in Understanding New Media:¹⁰