Mental Health Living Will Witnesses, Generally
Pennsylvania requires two subscribing witnesses to every Mental Health Living Will.  A Pennsylvania mental health living will that has not been properly witnessed will not be followed.

If the declarant personally signs the document, a witness may be any competent person who is at least eighteen years old.  If the declarant must ask another person to sign on his or her behalf, the person who writes the declarant's signature may not also sign as a witness.

In either case, the witnesses must be present and observe the declarant apply his or her signature to the document or observe the other person do so on the declarant's behalf.

See: Mental Health Living Will, Generally; Mental Health Living Will, Who May Create; Mental Health Living Will, Duration of Effect; Witnesses, Generally; Signing By Another Person; Signing By Mark