clark concedes that, in practice, the criterion of "equal efficiency" required by the parity principle is seldom met. he nonetheless proposes that the boundary of "skin and skull" is arbitrary and cognitively meaningless. if, in the example above, the paper and pencil used by the second person becomes a virtual paper-and-pencil visible on a monitor and controlled by a silicon chip implanted in the head, the similarity between the two situations becomes clearer.