Let's give Dan Ingalls the majority of the credit here. I will admit to "seeing" what was possible, but Dan was able to make really great compromises between what Should Be vs what would allow us to make great progress in the early 70s on relatively small machines (give Chuck Thacker the majority of the credit here for similar "art of the possible" with the Parc HW).

I liked the MOP book because they carried the model more deeply into the actual definitions -- I very much liked their metaphor that you want to supply a "region of design and implementation space" rather than a single point.