The debts of Marshall McLuhan to Harold Innis were many and are generally not well known. One of the more obscure ones must be a reference in Empire and Communications to Illusion and Reality (1937) by Christopher Caudwell (pseudonym of Christopher St John Sprigg). As was not unusual for McLuhan, he followed up Innis’ reference by reading Illusion and Reality and referenced it himself in ‘Notes on the Media as Art Forms’ (Explorations 2, 1954).

At least three things were highly important for McLuhan in Caudwell. First he reinforced the idea McLuhan was developing, perhaps especially from Joyce, that ‘language itself‘ is the decisive ground of experience (such that media had to be understood as languages). These are the  first lines of Illusion and Reality: 

This is a book not only about poetry but also about the sources of poetry. Poetry is written in language and therefore it is a book about the sources of languages. Language is a social product, the instrument whereby men communicate and  persuade each other; thus the study of poetry’s sources cannot be separated from the study of society.

Compare McLuhan in the same essay in which Caudwell is mentioned:

There has been very little discussion of any of these questions, thanks to the gratuitous assumption that communication is a matter of transmission of information, message or idea. This assumption blinds people to the aspect of communication as participation in a common situation. And it leads to ignoring the form of communication as the basic (…)¹ situation which is more significant than the information or idea ‘transmitted’. (…) The well-established view of culture which assumes that it filters down from élites to popular levels will not stand up for a moment to the facts of linguistic history and formation. Yet language is the great collective work (…)² transcending all individual works. Today this naive content-view of culture prevents us from directing serious critical attention to the media, old and new, as art forms. It is a charley horse inhibiting all education in a technological society. (‘Notes on the Media as Art Forms’)

Secondly, the citation from Caudwell reads as follows:

There is a poetic instant and as time vanishes, space enters; the horizon expands and becomes boundless.

That time and space might be related inversely (“as time vanishes, space enters”) became an increasingly important idea for McLuhan.³ Here Caudwell helped along what McLuhan already had from Innis. As well, the notion of the spatial horizon expanding and becoming boundless would come to serve McLuhan as a definition of Gutenbergian perspective and hence of the Gutenberg galaxy itself. 

Thirdly, Caudwell’s book was the first of a whole series read by McLuhan in the course of the 1950s dealing with ‘illusion and reality’. Ernst Gombrich’s Art and illusion was probably the most important of these, but all contributed to his notion of the difference between making and matching. If ‘matching’ were not even a possible goal on account of presence of illusion in all perception, what was the status of ‘making’? Did making produce only illusion? Or were making and truth somehow correlate despite an inevitable absence of matching? And were even truth and illusion somehow correlate themselves for ineluctably finite beings — who can yet figure things out?⁴