Comment by perrygeo
I respect all three authors deeply but this is my first time reading Tellman's take. His idea that "simplicity is not intrinsic ... Simplicity is a fitness between software and our expectations" has a very nice ring to it.
Contrast that with Hickey's notion of simplicity as an objective count of interleaved concerns or "folds". The quibble with this is, how do you delineate the concerns? Depending on your style and knowledge of the system, you'll get a different number. So it's hard to call it objective.
Tellman's definition is nice because it acknowledges the subjectivity, and puts it front and center. IOW the "style and knowledge of the system" form a mental model of the software system. What's important is not the cardinality, OR the interleaving. It's the ability for that model to make good predictions about the software's behavior. Accurate model held in the minds of humans that operate it == simple software.
The hickey definition is from the POV of the engineer - the creation of software with simplicity. Code that is simple, doesn't necessarily result in a piece of software that is simple for the end-user.
The tellman's version is for the end user - simplicity of use, by the user, with their existing expectation, culture and pre-knowledge. It's basically describing skeuomorphism in software (but not limited to just UI). It might be enormously complicated to create for the engineer, while it remains simple for the end-user.