Certain kinds of medical treatment are often held to be morally unacceptable because they are an 'interference with nature'. I suggest a way in which we can make sense of such ideas. We can make significant choices only against a background of conditions which we regard as 'natural', and these will typically include such facts as those of birth and death, of youth and age, and of sexual relations. I argue, however, that such ideas, though intelligible, do not establish any (...) valid moral objection to, for instance, the use of ovarian tissue for assisted conception. (shrink)