55. If we observe our impressions received through sensation, we shall perceive that they are all equal so far as the production of certainty is concerned. We are just as certain of the sensation caused by any noise whatever, as we are of that produced by an object which we see, an odorous body which we smell, a savory morsel which we taste, or any thing which strongly affects our sense of touch. There is no gradation in the certainty produced by these sensations: they are all equal; for if we speak of sensation itself, we experience it in such a manner as to leave no uncertainty; and if we speak of the relation of sensation with the existence of the object causing it, we are just as certain that the sensation called _sight_ corresponds to an external object _seen_, as we are that an external object _touched_ corresponds to the sensation called _touch_.
Hence we infer that no one sensation is the origin of the certainty of other sensations; in this they are all alike: and most men have no other reason than their experience why they should be sure of this certainty. We are aware that what happens to individuals from whose eyes cataracts have been removed, shows that simple sensation does not suffice for the due appreciation of the object perceived, and that one sense aids another: but this does not prove any one of them to be preferable; for as the blind man, whose sight was suddenly restored, did not form an exact judgment as to the size and distance of objects seen by sight only, but required the assistance of touch; so is it very probable that if a person of good eyesight had been deprived from his birth of the sense of touch, he would not be able, were this sense given him suddenly, to form an exact judgment concerning objects touched, until, by the aid of sight, he had become accustomed to combine the new and the old order of sensations, and learnt by practice to determine the relations of sensation with its object, or to know by sensation the properties of its object.
56. This fact of the blind man is however contradicted by others which lead to a directly opposite result. The youth, upon whom the oculist, Jean Janin, performed the same operation, and other persons blind from their birth, whose eyesight Luigi de' Gregori partly restored, did not, like the blind man of Cheselden, deem these objects stuck to their eyes, but that they saw them as things really external and separate. Rosmini thus relates it,[3] although he gives the preference to the Cheselden case, which he says was repeated in Italy by the professor Giacomo di Pavia with precisely the same results.