  There is a rough consensus in medical ethics on the requirement of respect for patient  autonomy: physicians must ultimately defer to patients' own decisions about the management of their medical care, so long as the patients are deemed to have sufficient mental capacity to make the decisions in question. For patients who lack the relevant decision-making capacity at the time the decision is to be made, a need arises for surrogate decision-making: someone else must be entrusted to decide on their behalf. Patients who formerly possessed the relevant decision-making capacity might have anticipated the loss of capacity and left instructions for how future medical decisions ought to be made. Such instructions are called an advance directive. One type of advance directive simply designates who the surrogate decision-maker should be. A more substantive advance directive, often called a living will, specifies particular principles or considerations meant to guide the surrogate's decisions in various circumstances, for example, “Do not prolong my life if I enter persistent vegetative state,” or “I am a fighter: do not discontinue life-sustaining treatment no matter what happens to me.”