Wright, Larry (1973). Functions. The Philosophical Review 82, 139–168.
We have seen that no matter how useful it is for X to do Z, or what contribution X‘s doing Z makes within a complex system, these sorts of consideration are never sufficient for saying that the function of X is Z. It could still turn out that X did Z only by accident. But all of the accident counterexamples can be avoided if we include as part of the analysis something about how X came to be there (wherever): namely, that it is there because it does Z-with an etiological “because.” The buckle, the heart, the nose, the engine nut, and so forth were not there because they stop bullets, throb, support glasses, adjust the valve, and all the other things which were falsely attributed as functions, respectively (p. 156).
When we give a functional explanation of X by appeal to Z (“X does Z”), Z is always a consequence or result of X’s being there (in the sense of “is there” sketched above). So when we say that Z is the function of X, we are not only saying that X is there because it does Z, we are also saying that Z is (or happens as) a result or consequence of X’s being there. Not only is chlorophyll in plants because it allows them to perform photosynthesis, photosynthesis is a consequence of the chlorophyll’s being there. Not only is the valve—adjusting screw there because it allows the clearance to be easily adjusted, the possibility of easy adjustment is a consequence of the screw’s being there. Quite obviously, “consequence of” here does not mean “guaranteed by.” “Z is a consequence of X,” very much like “X does Z” earlier, must be consistent with Z’s not occurring (p. 160).