McLuhan in a letter to Peter Drucker from December 15, 1959 (Letters, 259):

I refer to formal cause not in the sense of the classification of forms, but to their operation upon us and upon one another [of the forms themselves]. (…) Had a fascinating evening with Bernie Muller-Thym, last week, discussing these matters. He agreed with [the notion that] the entire order of existence and change becomes unintelligible if formal causality is banished from the center of study and awareness. At any rate, my media studies have gravitated toward the centre of formal causality, forcing me to re-invent it.

In Wholeness and the Implicate Order (1980)¹ Bohm discusses ‘formal cause’ in a way that illuminates McLuhan’s recourse to it:

It is of crucial significance (…) to understand (…) formal cause. Unfortunately, in its modern connotation, the word ‘formal’ tends to refer to an outward form that is not very significant (e.g. as in ‘formal dress’ or ‘a mere formality’). However, in the ancient Greek philosophy, the word form meant, in the first instance, an inner forming activity which is the cause of the growth of things, and of the development and differentiation of their various essential forms. For example, in the case of an oak tree, what is indicated by the term ‘formal cause’ is the whole inner movement of sap, cell growth, articulation of branches, leaves, etc., which is characteristic of that kind of tree and different from that taking place in other kinds of trees. In more modern language, it would be better to describe this as formative cause, to emphasize that what is involved is not a mere form imposed from without, but rather an ordered and structured inner movement that is essential to what things are. Any such formative cause must evidently have an end or product which is at least implicit. Thus, it is not possible to refer to the inner movement from the acorn giving rise to an oak tree, without simultaneously referring to the oak tree that is going to result from this movement. So formative cause always implies final cause.²(12-13, Bohm’s italics)