Synthesized answer
The provided passages do not explicitly state the central thesis of Marshall McLuhan's "Understanding Media: The Extensions of Man."
The passages identify the title of the book and its author, Marshall McLuhan [1]. However, they do not offer any summary or explanation of the book's core argument or main idea. Therefore, the central thesis cannot be determined from the given text.
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
Title: Rare Marshall McLuhan / Understanding Media The Extensions of Man 1964 - NY by Marshall McLuhan