Capacity¹ is at the heart of ethical decision making in healthcare. For those who subscribe to a principled approach to moral reasoning (Gillon, 1985; Gillon and Lloyd, 1993; Beauchamp and Childress, 1994), autonomy is often said to be pre-eminent amongst the four principles of medical ethics², or at least “first amongst equals” (Gillon, 2003). For those who prefer methods of ethical analysis other than the influential “four principles” approach, there remains a fundamental assumption that self-determination is afforded moral value and priority (Campbell, 2003; Harris, 2003; Cowley, 2005). To deny a capacitous person the freedom to make decisions about his or her own treatment and healthcare is fundamentally unethical whether one attributes the moral precept for such a conclusion to autonomy or virtuous practice or respect for others. Conversely, where a person lacks the capacity to make decisions, healthcare professionals have a responsibility to protect his or her interests³. It is remarkably easy to acknowledge the sense of the preceding statements; however, they belie the moral complexity of assessing capacity implying a satisfying but artificial binary certainty in which ethical practice constitutes an informed choice by the professional between affording his patient full entitlement to make his or her own decisions or acts without reference to the patient’s preferences because he or she has not met the demands of the “test” of capacity.