By Matthew A. Quick A health care power of attorney appoints an agent to act on a patient's behalf when he or she is unable, pursuant to a list of directives. On the other hand, a living will sets out a list of wishes, but does not give anyone the power to act on a patient's behalf (akin to a note to the doctor). Pursuant to Illinois law, if an agent is acting under a health care power of attorney, a living will is rendered inoperative. 755 ILCS 45/4-11.