In a 1933 paper for a philosophy seminar with Rupert Lodge at the University of Manitoba, McLuhan wrote:

In a universe constituted by inclusion, exclusion, limitation and participation,¹ there can be no principle of value; but if there is a nisus² of which these forms are an empirical expression and which is prior or superior to them, then it alone could be the principle of value or the unifying impulse of the universe.³

Almost 50 years later, in the last year of McLuhan’s life, here is Barry Nevitt (speaking as always more or less in McLuhan’s name) in ABC of Prophecy:

things properly understood are the visible manifestations of their invisible harmony.⁴

‘Empirical expression’ and ‘visible manifestations’ remain the same.  But a movement has been made away from the striving of nisus, or at least from a certain understanding of the striving of nisus, to perception of an existing “harmony”.

Nisus as a horizontal striving reveals no harmony, visible or invisible, precisely because it would reach something that remains outstanding and unrealized. However, nisus as a vertical striving, as the essential dynamic of all possibilities to realize themselves, is an existing harmony of possibility and actuality, of new and old, of creativity and reality. Nisus in these two senses is isomorphic with diachronic and synchronic times and the need (in general, but McLuhan’s in particular) was to move from the first of these to the second. Not to the second alone, however, but to the second in essential relation with the first — where the first would be the “empirical expression” of the second.

The great question is hinted at in McLuhan’s 1933 paper with its phrase, “a nisus (…) which is prior”. But “prior” in what sense? Diachronic or synchronic?⁵