text:  If then the processes which bring these cases within the same category with the rest require that we should assume the universality of the very law which they do not at first sight appear to exemplify is not this a real _petitio principii_? Can we prove a proposition by an argument which takes it for granted? And if not so proved on what evidence does it rest? "For this difficulty which I have purposely stated in the strongest terms it would admit of the school of metaphysicians who have long predominated in this country find a ready salvo