 Shepherd maintains that the evidence of the existence of sensations is immediate and incontrovertible; they are felt and their existence is nothing more than the feeling (EASP: 142). The conclusion of reasoning from the existence of sensations to that of some cause or other is equally certain. But inferences with regard to the existence of particular causes of specific kinds are liable to error both with regard to reasoning and lapse of memory. As a result, the evidence we have that a supposed object of a specific kind exists, e.g., an apple rather than a crab apple, consists of items of unequal evidential value. 