  Above, we noted that the traditional epistemological demand that one's evidence consist of that to which one has immediate and unproblematic access—and indeed, that one's evidence must be such that one can appreciate it (at least it in principle) even when one is in the most dire of epistemic predicaments—has often encouraged a phenomenal conception of evidence, according to which one's evidence is limited to one's experiences or sense data. On this picture, evidence consists of essentially private mental states, accessible only to the relevant subject. This picture of evidence stands in no small measure of tension with the idea that a central function of evidence is to serve as a neutral arbiter among competing views. For it is natural to think that the ability of evidence to play this latter role depends crucially on its having an essentially public character, i.e., that it is the sort of thing which can be grasped and appreciated by multiple individuals. Here, the most natural contenders would seem to be physical objects and the states of affairs and events in which they participate, since it is such entities that are characteristically accessible to multiple observers. (I ask what evidence there is for your diagnosis that the patient suffers from measles; in response, you might simply point to or demonstrate the lesions on her skin.) On the other hand, to the extent that one's evidence consists of essentially private states there would seem to be no possibility of sharing one's evidence with others. But it is precisely the possibility of sharing relevant evidence which is naturally thought to secure the objectivity of science. Indeed, it has often been held that inasmuch as the objectivity of science is underwritten by the fact that science is evidence driven, it is the public character of scientific evidence which is crucial. On this view, it is a central methodological norm of science to eschew as inadmissible (e.g.) any alleged episodes of incommunicable insight in considering whether to accept or reject a claim. The theme of the essentially public character of scientific evidence has been prominent from the earliest days of modern science—it was championed, for example, by Robert Boyle, founder of the Royal Society, who insisted that the ‘witnessing’ of experiments was to be a collective  act[36]—and  has remained so up until the present day. Unsurprisingly, the theme was taken up by philosophers of science with gusto. Thus, Hempel required that ‘all statements of empirical science be capable of test by reference to evidence which is public, i.e., evidence which can be secured by different observers and does not depend essentially on the observer’ (1952: 22). The idea was echoed by other leading positivists (see, e.g., Feigl 1953) as well as by Popper (1959). More recently, in the course of reviewing ‘some of the things objectivity, and specifically scientific objectivity, has been thought to involve’, Peter Railton singles out the idea that 