Plato’s Theory of Ethics by McLuhan’s mentor at the University of Manitoba, Rupert Lodge, must have been read by him, closely, in the early 1930s. There can be little doubt that meditation on Plato’s ‘mean’ as set out by Lodge is what eventuated in McLuhan’s ‘medium’ 25 years in the future.

Here are passages in page order from Plato’s Theory of Ethics on the ‘mean’:

Disease, then, is viewed by Plato as an upsetting of the balance, a disturbance of the mean or due proportion which holds the body together. 23

The body is (…) a whole, a single organism to the life of which each element contributes its part. It is held together by the “limit”, “mean”, or principle of proportion, with a unity resembling that of a musical composition. 23-34

The most that we can safely infer from Plato’s hints and metaphors is that excellence of memory is analogous to excellence of bodily condition generally, and depends upon the proper organization of the physiological factors concerned—i.e. upon the presence of order, law, and the “mean’’ and upon the absence of excess… 36

not only too little (…) but also too much is an evil, and (…) as is obvious in the case of eating and drinking (…) the mean state, something between the two extremes represents what is best 388-389

The avoidance of excess in any direction, and the seeking of the middle pathway as a guide through life is formulated by tragedian and historian long before Plato taught the doctrine of the “limit” and Aristotle the doctrine of the ‘‘mean” as expressing the secret essence of all excellence. To the Hellenic mind, this saying formulated that characteristic of sophrosyne which we call ‘‘moderation in all things” and temperance thus easily took on some of the attributes of a supreme good. 412

In many contexts (…) the ‘‘mean” and the “limit,” are highly praised as principles which make art and conduct valuable, and in a few contexts they are definitely stated to be the highest good. This is, of course, especially true in the Philebus, but the Laws are full of such contexts, and they are found scattered up and down all the later Dialogues, including the Republic. In order to obtain a concrete idea of what is to be understood by this term, we shall therefore deal with the whole field investigated in these later Dialogues, and shall inquire what is meant by “the mean” in the world studied by natural science, in medical science and art, in the art of music, and in human conduct. From such an investigation we should be able to discover the general sense of the term, and in particular to discover why it is identified with the highest good. 442

the mind applies its own fundamental principle of the “limit” or ‘‘mean” to an otherwise unlimited and chaotic “extension” and “motion”. The principles and details of the theoretical universe thus constructed derive their whole significance from the principle of the “mean”. 442

What occurs in the absence of the mean? (…) The various elements refuse to combine, and fight with one another. 444

absence of the mean is synonymous with disintegration, chaos, not-being, while the presence of the mean signifies proportion, law, order, control, and reason. 445

both artist and philosopher are guided throughout by the principle of the mean. It is this principle which [in music] introduces unity, law, and order into the individual notes, the individual scales, the individual rhythms, and the composition as a whole (…) In what follows [from this this in theory], the artist does not need to understand; but it is fully apprehended by the philosopher. It is because the principle of the mean is also the principle of order (…) in the actual cosmos 453

The “idea” [= manifestation] represents the application of the principle of the mean or limit to a specific field of experience, so that to observe the mean is to follow the principle of the idea (…) that (…) has (…) a metaphysical basis (…) formed upon the objective principle of ultimate reality. 454

when it is stated that “the mean” is the highest good, what is meant is that the principle of the idea is the source of all idealization, of all elevation of fact to value, in a word, that the principle of the mean is identical with the principle of the (…) idea of good,  455