Thus we are treated to excursuses on what it is to divide a class at places where there are real subclasses marked off by their own specific natures, not in some arbitrary and merely conventional way; on which sorts of things require the preliminary study of 'models' in order to understand them fully and on the precise nature of such a 'model'; on the often neglected but crucially important science of measuring things by reference to 'due measure' and not relatively to given other things exceeding or falling short of them in the relevant respect length, weight, size, etc.