Harman points to Figure-Ground Communication's interview with Eric McLuhan. It includes a question from Harman about Laws of Media, namely "why did they they the tetrad to human artifacts?" Of course, this is also the question Levi and I will pose in our planned book on McLuhan.

McLuhan doesn't really answer the question, from the very beginning seeming not to take it at face value ("Graham, you are putting the cart somewhat before the horse, in order to be provocative--as I know you are aware.") But of course Graham's question is completely earnest and straightforward.

Come to think of it, I can't help but chuckle at Eric McLuhan's answer. Indeed, Harman's suggestion is precisely to put the cart and the horse before the human driver, if just for once.

I'll leave it to you to read the rest of the response, which is as extensive as the interview in general.