> the mental model that allows the designer to map a subset of the world (the domain) to and from the system, and not the system itself, is the primary product

> the mental model that allows the designer to map a subset of the world (the domain) to and from the system, and not the system itself, is the primary product of the software design activity This is spot on. I was never able to put it in such precise words. This theory has Brook's Law as a corollary: "Adding manpower to a late software project makes it later." Because the developers need time to develop this mental model before they can meaningfully contribute to the codebase.