One might try to meet this objection by revising the Via Negativa so that what is intended is only a partial definition along these lines: F is a physical property only if F is non-mental. Even so problems remain. As we have seen élan vital causes a problem because it is neither mental nor physical. But there might be properties that are both mental and physical. Consider a version of the identity theory according to which being in pain just is c-fibers firing. If we suppose that such a theory i