"The expressive possibilities of collage seem so simple that one is tempted to think that anyone could employ them to equal effect. Yet when one reviews the works of this early period - the printer's plate prints, say, those compositions made with the aid of old line blocks found in a printer's shop - it becomes obvious that Max Ernst's brilliant accomplishment consisted of having developed a syntax by which the employment of this found material could be controlled."
"Again, the crux is this: Max Ernst's careful selection of seminal imagery employed in collages and all the variants of collage, and the formal criteria which determined the composition of the printer's plate prints, rubbings, overpaintings, montages of photographic positives and paste-ups of wood engravings all indicate the primacy of control. Everywhere we look, we find invariables that oppose the seemingly unlimited availability of the material, that place considerable restrictions on its character and use."
Text from Werner Spies, introduction to "Max Ernst: A Retrospective"
I found this text on Mark Harden's Artchive