Voluntary Abortion involves the preventable destruction of an unformed or partly formed human organism. It is disagreement about the moral significance of this fact which sustains the controversy over abortion. This disagreement derives from a dispute about the sanctity, or morally protected position, of the destroyed organism, which in turn results from conflicting opinions about the basis or criterion of sanctity rather than any uncertainty over how they may apply. The thesis of this paper is that this last disagreement about the criterion of moral sanctity cannot be settled by rational, i.e. non-circular, argument; and hence, that there is no rational method of resolving the controversy over the morality of abortion.