Marchand, The Medium and the Messenger:

There were times when McLuhan felt proud that he had been an intellectual pioneer, almost the first person in the West since Plato, as he sometimes put it, to study effects rather than to talk about causes. (278)

McLuhan was overstating the case, of course.  Gilson and Muller-Tyme had taught him how important this method was to Augustine and to Christian philosophy generally. But the claim gives good measure of the importance within McLuhan’s work of this sort of investigation. In the sciences it is comparable to a chemist recommending close attention to the workings of particular material reactions in order to illuminate the table of elements. Or to a physicist attempting to define a law through concrete observations of, say, the path of a planet. But in the humanities, far more consequentially, it is the one way around nihilism.

Added April 30, 2023: In the 1940s McLuhan had the idea from Poe and Eliot that the artist must start with the effect she wants to achieve with an artwork and then figure out the steps leading to that effect. Those steps would be the artwork as an intro-duction to the desired effect. Or of the sparking of it. Later he realized that this idea could be turned around with the aim of achieving another effect rather than the starting one. For example, if nihilism were taken as an effect, or war, or social discord, or despair, figuring out the steps leading to that effect could expose how to avoid it by changing those steps. This notion that all problems are achieved effects that are avoidable once they are investigated as effects is one of the important aspects of of McLuhan’s faith:

One of the advantages of being a Catholic is that it confers a complete intellectual freedom to examine any and all phenomena with the absolute assurance of their intelligibility. (McLuhan to Martin Esslin, September 23, 1971, Letters 440)