In the debate around universals, Quinton defended a variety of nominalism that identifies properties with a set of "natural" classes.David Malet Armstrong has been strongly critical of natural class nominalism: Armstrong believes that Quinton's 'natural' classes avoid a fairly fundamental flaw with more primitive class nominalisms, namely that it has to assume that for every class you can construct, it must then have an associated property. The problem for the class nominalist according to Armstrong is that one must come up with some criteria to determine classes that back properties and those which just contain a collection of heterogeneous objects.
Quinton's version of class nominalism asserts that determining which are the natural (property) classes is simply a basic fact that is not open to any further philosophical scrutiny. Armstrong argues that whatever it is which picks out the natural classes is not derived from the membership of that class, but from some fact about the particular itself.
While Quinton's theory states that no further analysis of the classes is possible, he also says that some classes may be more or less natural—that is, more or less unified than another class. Armstrong illustrates this intuitive difference Quinton is appealing to by pointing to the difference between the class of coloured objects and the class of crimson objects: the crimson object class is more unified in some intuitive sense (how is not specified) than the class of coloured objects.
The shortest definition, and it is quite a good one, is that philosophy is thinking about thinking. That brings out the generally second-order character of the subject, as reflective thought about particular kinds of thinking – formation of beliefs, claims to knowledge – about the world or large parts of it. — The Oxford Companion to Philosophy, p. 666 (1st ed.)
Philosophy is rationally critical thinking, of a more or less systematic kind about the general nature of the world (metaphysics or theory of existence), the justification of belief (epistemology or theory of knowledge), and the conduct of life (ethics or theory of value). Each of the three elements in this list has a non-philosophical counterpart, from which it is distinguished by its explicitly rational and critical way of proceeding and by its systematic nature. Everyone has some general conception of the nature of the world in which they live and of their place in it. Metaphysics replaces the unargued assumptions embodied in such a conception with a rational and organized body of beliefs about the world as a whole. Everyone has occasion to doubt and question beliefs, their own or those of others, with more or less success and without any theory of what they are doing. Epistemology seeks by argument to make explicit the rules of correct belief formation. Everyone governs their conduct by directing it to desired or valued ends. Ethics, or moral philosophy, in its most inclusive sense, seeks to articulate, in rationally systematic form, the rules or principles involved. ibid