Perhaps neither, I think. As I understand it, is the concept of medium
in a sense
prior to the distinction between objective (physical) world
and subjective (internal) model of it. IN a sense, I
read it as comparable to Merleau-Ponty's concept of 'Flesh'
(I am not sure if you like that one!;-))
Here is a passage from Maturana, 1987, p. 340; perhaps
you may come to an entirely different interpretation than
I do!:
"I call medium of a unity the containing background of
distinctions, including all that is not involved in its
structure if it is a composite one, (.....) in which it realizes its
domain of existence. The medium includes both that part of the background
that is distinguished by the observer as surrounding
the unity, and that part of the background the observer
conceives as interacting with it, and which it obscures in its
operation in structural coupling (in its domain of existence).
I call this latter part of the medium oeprationally defined
moment by moment in its encounter with the medium
in structural coupling, the _niche_ of the unity.
Accordingly, a unity continuously realizes and specifies its
niche by actually operating in its domain of perturbations
while conserving adaptation in the medium."
Best wishes,
Arno Goudsmit