 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 is true, is the property of being in pain then mental or physical? Both presumably; but this could not be true on the Via Negativa construed as a definition of what a physical property is, even a partial definition. For if a property is mental and physical, then, given the Via Negativa, it will be both mental and non-mental which (of course) it can’t be! Now obviously, there are good questions about whether an identity theory along these lines is or could be true, but regardless of whether it is true, it should not be ruled out simply because of a proposal about how to define the words in which it is stated. 