As for the courts that are called upon to settle certain cases, they will need some political guidance or governing principles to do so. For example, what if a tenured professor of bioethics, unable to bear the loss of his cognitive powers, leaves written instructions not to treat any infections if he ever suffers dementia? Decades later, now suffering from Alzheimer's, the former professor is mentally impaired but seemingly happy. He can't recognize his children, but he seems to enjoy the sunset. He's been physically healthy for years, but then gets a urinary tract infection. All his family members believe he should be treated.
Should the state intervene to prohibit antibiotics--to protect the incompetent person's "right to die"? Or should the state leave the family members alone, so they can do what they believe is in the best interests of the person the professor now is? If Andrew Sullivan and other critics are worried about "theocons" using the power of the state to undermine the right to self-determination, are they willing to use the power of the state to impose death when families choose life? Is this what their idea of "autonomy" really requires?
I'm with Andrew on this one: my hard-hearted libertarian self says, "Yes." Here's where I lay out my reasoning.
There is no consensus on when exactly a person is dead. It is true that a person can not be alive without a heart beat, but that does not mean that a person with a heart beat is alive. If (no A), then (no B) does not reverse to: if (A), then (B). Only when there is an absolute indicator that only happens when a person has passed on can a standard be set. Anything else would be to confuse life with its trappings. Where there is no scientific or legal standard as to when "alive" ends it must ultimately rest with the conscience of the individual.
It is appropriate for the state to set legislatively what the value of "no clear intention" should be. A good default would be to maintain what life is left. It is then the responsibility of the indvidual to make his/her choice clear or choose a trusted person who will act on his/her behalf.
What happens if a person makes an unwise choice in who to trust or how the document was worded? Hard-hearted, but "Too bad". Just add this to the list of things that will bring you to grief, or worse, if you do not take appropriate care of yourself.