Mass. Gen. Laws ch. 201D § 2
Every competent adult shall have the right to appoint a health care agent by executing a health care proxy. Said health care proxy shall be in writing signed by such adult or at the direction of such adult in the presence of two other adults who shall subscribe their names as witnesses to such signature. The witnesses shall affirm in writing that the principal appeared to be at least eighteen years of age, of sound mind and under no constraint or undue influence. No person who has been named as health care agent in a health care proxy shall act as a witness to the execution of such proxy. For the purposes of this section, every adult shall be presumed to be competent and every health care proxy shall be presumed to be properly executed unless a court determines otherwise.
A competent adult may designate an alternate health care agent as part of a valid health care proxy. Said alternate may serve when the designated health care agent is not available, willing or competent to serve and the designated health care agent is not expected to become available, willing or competent to make a timely decision given the patient's medical circumstances; or, the health care agent is disqualified from acting on the principal's behalf pursuant to other requirements of this chapter.
Mass. Gen. Laws ch. 201D, § 2