 McMahan (1993) picks out three key factors which he claims affect whether withdrawal of a barrier counts as doing or allowing “...whether the person who terminates the aid or protection is the person who has provided it, whether the aid or protection is self-sustaining or requires more of the agent, and whether the aid or protection is operative or as yet inoperative” (McMahan 1993, 262). Withdrawal of a barrier counts as merely allowing harm if and only if the barrier was (a) provided by the agent and the barrier is either (b) not self-sustaining or (c) not yet operative. Thus McMahan’s account seems to correctly classify Impoverished Village and Hospital as merely allowing harm and Burning Building (Enemy) as doing harm. It may, however, counterintuitively classify Gallows as merely allowing harm if the trap door belongs to the agent. If the trap door belongs to the agent, the continued use of his resources is required to keep the barrier in place, so the barrier may not count as self-sustaining. McMahan also allows that sometimes one agent can act on behalf of another, or they can act as a team. This is supposed to explain the intuition that in the Burning Building case where the net is moved to catch five imperilled persons instead of one, it doesn’t matter whether it is the original firefighter or a second firefighter who does the  moving.[26] 