If we wish to deny everything that is not sensible, then we must deny the realities which unquestionably exist. For example, ethereal matter is not sensible, though it has an undoubted existence. The power of attraction is not sensible, though it certainly exists. From what do we affirm these existences? From their signs. Thus this light is the vibration of that ethereal matter, and from this vibration we infer the existence of ether.