A cluster concept is one that is defined by a weighted list of criteria, such that no one of these criteria is either necessary or sufficient for membership. Wittgenstein alleged that game was such a concept; some have claimed that species concepts are cluster concepts. (Homo sapiens was once defined as featherless biped, but there are human amputees who are not (actually) bipeds; moreover evolution may some day give us feathers (or analogues thereof).
It was at one time pretty commonplace to think that cluster concepts are quite common in science and culture. People say, for instance, that democracy is a cluster concept; Denis Dutton has recently argued that art is a cluster concept. I disagree. In my view, clusters are needed for what I call "diagnostic definitions": definitions that tell you how to recognize a concept-instance. But I would argue that despite the surface variability, there is an underlying unity in the sorts of concept that are robustly used in everyday and scientific discourse.
The classic articulation of this notion was of course Wittgenstein's in Philosophical Investigations I 66: "Consider for example the proceedings that we call 'games'. I mean board-games, card-games, ball-games, Olympic games, and so on. What is common to them all? -- Don't say: "There must be something common, or they would not be called 'games'" -- but look and see whether there is anything in common to all. -- For if you lok at them you will not see something that is common to all, but similarities, relationships, and a whole series of them at that." Some games involve cards, others boards, yet others balls; some involve winning and losing, others (like ring-a-ring-a-roses, or a child bouncing a ball off a wall) do not. "Look at the parts played by skill and luck; and at the difference between skill in chess and skill in tennis." And so on.
The classic refutation of this particular claim was due to Bernard Suits. He defined a game as an activity in which you accept certain rules that limit how you can achieve a certain activity. "To play a game is to engage in activity designed to bring about a specific state of affairs, using only means permitted by specific rules, where the means permitted by the rules are more limited in scope than they would be in the absence of such rules, and where the sole reason for accepting the rules is to make possible such activity." In golf the "specific state of affairs" aimed at is that a ball wind up in a hole: but you limit how you will achieve this end in order to play the game.
The difference between Wittgenstein and Suits is while the former looked at ways for an observer to identify games, Suits articulated an underlying commonality that is unobservable to a casual observer. Wittgenstein's definitions were diagnostic; Suits was after the real essence.
The same is, of course, true of species. The theory of natural selection demands that there be variability among members of a species, both at a time and through evolutionary history. This is an obstacle faced by those who would give a diagnostic definition of species (or what naturalists sometimes call a "key" -- the sort of thing you find in bird-watching books -- if you think a key is a diagnostic definition). But historical definitions of species, i.e., definitions that define species in terms of common origin, or definitions that define them in terms of reproductive isolation give unified definitions in terms of non-observables.
My thesis is that no cluster concept -- no concept primarily defined by a cluster -- plays an explanatory role in science or everyday discourse. (I would include social science under science, and aesthetics under everyday discourse -- so the scope of my thesis is quite wide.) I would spice up my thesis by adding: no non-explanatory noun-phrase plays any important role in scientific or everyday discourse. The "spicing up" is just a way of saying that you hardly ever come across a cluster concept. But this is an add-on.
Do people think that this thesis is obvious, vacuous, false, inflammatory . . . what?