In the last decade the use of advance directives or living wills has become increasingly common. This paper is concerned with those advance directives in which the user opts for withdrawal of active treatment in a future situation where he or she is incompetent to consent to conservative management but where that incompetence is potentially reversible. This type of directive assumes that the individual is able accurately to determine the type of treatment he or she would have adopted had he or she been competent in this future scenario. The paper argues that this assumption is flawed and provides theoretical and empirical evidence for this. If the assumption is false, and those taking out advance directives do not realise this, then the ethical bases for the use of these advance directives -- the maximisation of the individual's autonomy and minimisation of harm -- are undermined. The paper concludes that this form of advance directive should be abolished.