Your question suggests that answers to the question “What is thefunction of X?’ will have normative implications about what we ought orought not, may or may not, do to Xs. And this fact is puzzling. How,you might be wondering, can certain facts about an object’s functionhave any implications about what I may or may not do to it? And I thinkthat you are right to be skeptical: in the case of ashtrays, functionaldefinitions have no normative implications for us– about what we may ormay not do to them.
However, behind your question may be Aristotle’s “function argument” in the Nicomachean Ethics (I 7), where he argues that information about the “function” (ergon) of humans has implications for what sort of life humans ought to live-- “ought”, that is, if they are going to be well off.
Onmy view, Aristotle’s notion of function does not correspond to any ofthe three notions of function that Nick distinguishes for ashtrays. Todistinguish Aristotle’s notion from those that Nick defines, I’ll referto it as an object’s “real function.” On Aristotle’s view, the notionof an object’s real function plays a significant explanatory function. If I want to know why this hammer has the attributes that it has, I have to understand what the hammer is for. If I want to know why hearts have the attributes they have, I have to know what they are for.Hammers and hearts can be used for all sorts of purposes, but not allof these purposes will be equally useful for such explanatory purposes.
The fact that we put objects to uses other than what they arereally for usually doesn’t much matter– sometimes we can put objects tobetter use than we could if we restricted ourselves to using them onlyfor real function. This statue is quite hideous, but it makes for avery effective doorstop. On Aristotle’s view, there is a significantexception to this general rule. While we might put other objects tobetter use than what they are really for, our own selves are adifferent matter altogether. On Aristotle’s view, if we are going tolive the best life, we must perform our real function and do so well.