One way of doing moral philosophy is to restrict yourself to analyzing the moral concepts we use. Instead of asking “why ought we to be just”, ask “what is justice anyway?” What kind of situations do we generally call just or unjust? What type of actions elicit moral approval or disapproval? What is “blame”, after all, and what prompts it to rear its head? These kind of questions, I want to argue, are an attempt to discover natural kinds in the way we think about ethics.
A natural kind, roughly speaking, is a way of classifying things that “carves the world at its joints”. Obviously “tiger” only exists as an abstract concept, as a word signifying a collection of animals that share certain properties. But – for complicated reasons involving genetics – we consider “tiger” to be more accurate than, say, a word that collects all animals that happen to be facing north at this particular moment. We generally say that the second word is arbitrary, while “tigers” do actually exist in the real world.
In the same way we consider some ways of judging moral actions to be arbitrary, while others less so. Surely the colloquial sense of “freedom” does refer to some coherent, valuable thing in the real world (although many of us might consider Ayn Rand’s brand of freedom differently). There is useful philosophical work to be done in teasing out exactly what we refer to when we use words like freedom, justice and equality – or to put it another way, why concepts like “pain” and “consent” are morally significant but concepts like “hair color” are not.
Of course, this line of inquiry gets us nowhere with the normative element of moral philosophy. Even if we can set out exactly what we mean by justice, there’s no guarantee that a “just” action is actually an action we ought to perform. However, breaking apart concepts like justice can be very productive. Jade (the precious stone) is the canonical example of where natural kinds are useful. Jade can refer to two kinds of rock – jadeite and nephrite – which, from a chemical perspective, are very different. It is clear that, for a chemist, “jade” simply does not work as a natural kind: it is necessary to zero in on the precise nature of the mineral.
Likewise in moral philosophy. If we establish (for instance) two distinct types of ‘justice’, it will become necessary to ask ourselves which kind we are using in any particular moral decision – which may, like the chemist working with nephrite, lead us to avoid common moral mistakes.