Everybody allows that to know any other science you must have first studied it, and that you can only claim to express a judgment upon it in virtue of such knowledge. Everybody allows that to make a shoe you must have learned and practised the craft of the shoemaker, though every man has a model in his own foot, and possesses in his hands the natural endowments for the operations required. For philosophy alone, it seems to be imagined, such study, care, and application are not in the least requisite