  However natural such a picture might be, it is at least somewhat difficult to reconcile with historically prominent philosophical accounts of the nature of evidence. Russell, the greatest empiricist of the first half of the twentieth century, tended to think of evidence as sense data, mental items of one's present consciousness with which one is immediately acquainted. In this, he stood squarely within the tradition of classical empiricism. Quine, the greatest empiricist of the second half of the century, maintained throughout his career that evidence consisted of the stimulation of one's sensory  receptors.[2]  The logical positivists held that whatever evidence there is for a given scientific theory is afforded by observation statements or ‘protocol sentence’, linguistic entities with suitably-restricted contents; the precise nature of the restrictions became a vigorously contested matter within the tradition  itself.[3]  According to one recent and influential study, one's evidence consists of the totality of propositions that one knows (Williamson 2000). According to another, one's evidence consists exclusively of one's current mental states (Conee and Feldman 2004). Within contemporary confirmation theory, a prominent version of Bayesianism is naturally understood as identifying one's evidence with those beliefs of which one is psychologically certain. Of course, the suggestion that one might place sense data, sensory receptor stimulations, known propositions, or one's current mental states in a plastic bag (or dig them up from the ground, or send them to a laboratory, or …) is of dubious intelligibility. From the perspective of much ordinary thought and talk about evidence, much philosophical theorizing about evidence would seem to embody a particularly grotesque category mistake. 