  On what is perhaps its most natural reading, the truth of this claim was an empirical discovery of medical science. At a certain point in time, it was discovered that Koplik spots are a reliable indication of measles—something which was true, presumably, long before the discovery in question. Here, the evidence relation is understood as a relation that either obtains or fails to obtain independently of what anyone knows or believes about its  obtaining.[17]  To the extent that one is concerned to arrive at an accurate picture of the world, knowledge of instances of this relation—roughly, knowledge of what bits of the world tend to accompany what other bits of the world—would seem to be exactly the sort of thing which one is seeking.  When the evidence relation is construed in this way, investigating it is of a piece with investigating the world itself.