It is plain, I think, that if we have any reason at all for supposing that in all probability Alexander the Great did see an elephant, and that in all probability no such person as Sindbad the Sailor ever saw a Roc, part of that reason consists in the assumption that some other person would probably not have had just those perceptions which he did have, unless Alexander the Great had seen an elephant, and unless Sindbad the Sailor had not seen a Roc.