People often ask ill-conceived questions about whether, by using things like modern medicine, we obstruct natural selection. My own thought is that this thinking is facile, because it presumes that either medicine or caring for sick people are somehow "unnatural" interventions for our species. Animals like chimpanzees are known to use tools, and we wouldn't say that is unnatural for them--isn't medicine just a human tool? But then I wonder if conceiving of natural selection in this way makes it impossible to say that we ever obstruct natural selection, which seems like an logically odd consequence. I was wondering if there is a more discriminating formulation of natural selection according to which we obstruct natural selection in some cases and not others.