[voidifremoved]

But then both are just a kind of polygon, with an arbitrary number of sides of arbitrary length.

At which point you really want a variable length list of Sides, each with a length property.

Or you want a series of points with relative coordinates, making the sides implicit.

Or or or...

What I take from it is that there is no one true object model, there is no universally "correct" way of solving the problem. An evolving understanding of the problem leads to an evolving solution that places different priorities on different attributes.

Is there any value provided to the system by making Square a specialisation of Rectangle? Is there value in being able to express a Square without a redundant attribute value? What is the tradeoff? And so on.

[unclebucknasty]

It's interesting because it highlights that if a thing has so many properties describing what it is, then at a certain point it's not a thing at all, but a collection of characteristics that could be tweaked to describe anything.

At that point, the idea of behaviors is rendered void, as the universe of possible behaviors is just too large.

So, I think that's the guiding principle in your suggestion to consider what value a given design provides to the system: that is, start by modelling the behaviors your system requires, then create an object hiearchy that reflects those behaviors most concisely.

This is probably our intent, but we may be derailed by too little respect for YAGNI.

[dwaltrip]

This is a very insightful point. These models and concepts are all human created abstractions.

They contain elements of arbitrariness, describe different levels of detail, focus in on certain aspects or dynamic, leave out other information, and so on.

The skilled software developer must determine how useful and well-suited any particular abstraction is for the actual problem at hand, and judiciously move forward that.

As understanding of the problem domain grows, the abstractions should be updated and discarded to reflect this.

Pragmatism is key -- what does the system actually need to do? Supposed platonic ideals of some conceptual object are often a wild goose chase. Reality is complex. We must embrace it.