If the patient then raises his arm, then that is proof-positive that it is possible for him to do so. And equivalently, Possible truth is a necessary condition for actual truth. Table salt dissolves in water. Pierre Trudeau was the first Canadian to travel in a space capsule. There are three times as many women in Canada as there are men.
All aunts are female.
Some pigs can levitate. All aunts are female. Whatever is blue is colored. There are either fewer than 20 students in the room or there are more than 10. It is false that some triangle has exactly four sides.