For many ages it has been allowed by sensible men, Nihil est in intellectu quod non fuit prius in sensu: That is, There is nothing in the understanding which was not first perceived by some of the senses. All the knowledge which we naturally have is originally derived from our senses. And therefore those who want any sense cannot have the least knowledge or idea of the objects of that sense; as they that never had sight have not the least knowledge or conception of light or colours.