 Is Aristotle an explanatory empiricist? Aristotle does accept the need for inferences from what can be observed to facts that are not immediately evident (Asmis 1984: 212–17). As he writes, “to gain light on things imperceptible we must use the evidence of perceptible things” (EN 1104a13–14). It follows that something perceived can serve as a sign of something that we would not otherwise know. Roughness of the tongue, for example, is a sign (sēmeion) of fever (DS 462b26–32); the large size of hailstones is a sign (sēmeion) they have been formed close to the earth (Meteor. 348a33–36). But while knowledge of signs constitutes evidence, it does not, in themselves, represent scientific knowledge. Scientific knowledge is knowledge not just of facts, but of their causes. Even if the association between sign and signified is without exception, so that one can infer the latter from the former, such an inference is not the same as offering an explanation. It may be, for instance, that fever is a reliable sign of illness (Rhet. 1357b18–19): all those with fever have an underlying medical condition. But this does not yet constitute an explanation. Clearly, the fever does not explain the illness. While a particular illness could explain the fever, an explanatory inference of this kind would go in the other direction: it would take us from the illness to the fever, showing the latter to be a necessary consequence of the former. A sign, in other words, can establish only what Aristotle calls “the fact” (to hoti); showing a correlation between sign and signified does not, in itself, establish the “reasoned fact” (to dioti) (Madden 1952: 372; cf. APo. 78a23).