In The Gutenberg Galaxy McLuhan reformulates the Aristotelian-Thomist concept of the sensus communis in terms of a 'sense-ratio', a concept ostensibly adopted from William Blake.
He says that Blake's theme is captured in these lines from Book II, Chapter 34 of the poem:
If Perceptive organs vary, Objects of Perception seem to vary;
If the Perceptive Organs close, their Objects seem to close also.
McLuhan links this with the concept of 'ratio' which appears later in the poem (Book III, Chapter 74):
The Spectre is the Reasoning Power in Man, & when separated
From Imagination and closing itself as in steel in a Ratio
Of the Things of Memory, It thence frames Laws & Moralities
To destroy Imagination, the Divine Body, by Martyrdoms & Wars.
He explicates Blake's concept of 'Imagination' (an Aristotelian concept that was popularized by Romantic poets such as Blake, Samuel Taylor Coleridge and John Keats) using the concept of 'synesthesia' to be found in E. H. Gombrich's Art and Illusion: A Study of the Psychology of Pictorial Representation (1960).
Gombrich describes 'synesthesia' as 'the splashing over of impressions from one sense modality to another.... [for example] [t]here is touch in such terms as "velvety voice" and "a cold light," taste with "sweet harmonies" of colors or sounds, and so on .... Synesthesia concerns relationships'.2
Relating this to Blake, McLuhan says: 'Imagination is that ratio among the perceptions and faculties which exists when they are not embedded or outered in material technologies.'3
A 'closed system', meanwhile, is the technological 'extension' of a sense that is 'incapable of interplay' with the other senses.4
In fact the concept of the 'closed system', while McLuhan attributes it to Blake, has clearly been derived from systems theory, and specifically from Kenneth Boulding's The Image: Knowledge in Life and Society (1956), where the 'open system' is described as 'a structure ... which is continually taking in something from its environment and giving out something to its environment, all the while maintaining its structure in the middle of this flow', while the 'closed system' designates inorganic matter.5
As McLuhan puts it, 'the extension of one or another of our senses by mechanical means ... can act as a sort of twist for the kaleidoscope of the entire sensorium. A new combination or ratio of the existing components occurs ...'6
Man 'is then compelled to behold the fragment of himself "closing itself as in steel."'7