McLuhan takes from St. Thomas Aquinas the idea of 'consciousness' as that power by which 'all sensible qualities are related'.1
Aristotle argues for a 'common sensorium' or 'common sense' in De Anima or On the Soul (c. 350 BC), where he says that each sense 'is relative to its particular group of sensible qualities: it is found in a sense-organ as such and discriminates the differences which exist within that group; e.g. sight discriminates white and black, taste sweet and bitter, and so in all cases'.2
He then asks: 'Since we also discriminate white from sweet, and indeed each sensible quality from every other, with what do we perceive that they are different? It must be by sense; for what is before us is sensible objects...'
He concludes that 'Both the discriminating power and the time of its exercise must be one and undivided.'
In his commentary on De Anima, Aquinas says that the perceptions must be unified by means of a sensus communis (the Latin translation of Aristotle's term) or 'common sense', which Aquinas (and McLuhan after him) suggests is that of 'touch'.
Aristotle raises the problem of whether touch is 'a single sense or a group of senses' in De Anima; he says that 'flesh is not the organ but the medium of touch', and that the organ of touch must be located where the object of touch is perceived, i.e., the place where the senses meet together.3
He says that 'without touch it is impossible to have any other sense .... Touch takes place by direct contact with its objects .... All the other organs of sense, no doubt, perceive by contact, only the contact is mediate: touch alone perceives by immediate contact.'4
Accords Aquinas: 'the sense of touch is generically one, but is divided into several specific senses, and for this reason it extends to various contrarities .... We might also say that all those contrarities agree, each in some proximate genus, and all in a common genus, which is the common and formal object of touch.'5
In his books of the 1960's, McLuhan formulates the interplay of the senses in terms of 'tactility', which he says 'is not a sense but an interplay of all senses'; and from 1963, he relates 'tactility' or the sense of 'touch' to the function of the central nervous system.6
McLuhan suggests that while we are used to thinking of 'touch' as a function of the skin, its function is rather the 'contact', 'interplay' or communication of senses - it is 'the very life of things in the mind'.7