Wittgenstein vindicated - types are not a hierarchy

In the days when object-orientation was new to most programmers, we were occasionally surprised by apparently profound failings of metaphysical conception among our colleagues. In one case that was notorious among members of our team for a long time, an early draft of a programming tutorial described inheritance in terms of the parts of a car - the class "car" should inherit the class "wheel", the tutorial explained.

Wittgenstein might have observed a problem in the metaphysics of object-oriented programming, rather than an ontological disconnect in the life of that author (and of every person who had read that draft before I saw it). Is it really the case that we always know what category is a kind of what other category?

Well, I spent most of today resolving a similar conceptual reversal. Until today, I had assumed that "content" was a kind of "layer". Now I've decided that layer is a kind of content. Of course the real problem is that neither is fully a kind of the other. Most development projects involve epistemological compromises, and expert users of the software we create (if the category terms are revealed) manage to subtly change their previous understanding of those words, in that context, to anticipate the actual system behaviour. Unfortunately, when programming in Java, reversing the position of two classes in the inheritance hierarchy is far from a trivial exercise.