… is from page 214 of Vol. 19 (Ideas, Persons, and Events ) of The Collected Works of James M. Buchanan; specifically, it’s from Jim’s 1997 article “The Epistemological Feasibility of Free Markets” (footnote omitted):
[T]he market is not to be conceived as analogous to a mechanism, but, instead, as a process within which separate persons interact in furtherance of their own purposes and, in so doing, generate an order. This order of the market may, of course, be described in terms of its results by a vector of valued inputs and outputs, but this vector itself emerges from the process and does not, and cannot, exist independently of the process through which it is generated.
The omitted footnote from the above quotation is to the greatest economics article ever written when measured in terms of importance and depth of ideas relative to the number of words used to convey those ideas – namely, Jim’s 1982 note “Order Defined in the Process of its Emergence.”