"All human artifacts--and the extensions of man--are
patterned structurally in the mode of the word. Whether
it is a medium like a radio, a bulldozer or a safety pin,
whether it is the work or a law of science, all these 
utterings and outerings of man have a four-part
structure, which is that of metaphor itself. I will
illustrate this discovery from the character of money
which: a)enhances the speed of exchange; b)obsolesces
barter: c)retrieves potlatch; and)...flips into credit."
(Avant-Garde Magazine April 1978)

For more see the" Laws of Media" McLuhan and McLuhan
Univ. of Toronto Press,1988