 While we know of the medical empiricists only indirectly, we do have first-hand access to the thought of another group of explanatory empiricists, namely the sceptics of the Pyrrhonian school as represented by the writings of Sextus Empiricus (ca. 150–225 CE). (The empiricus title suggests that he, too, was associated with the medical writers.) Worth noting, first of all, is Sextus’s presentation of the problem of induction (epagōgē). This assumes that epagōgē is simply a process of enumerative induction. As we have already seen  (sect. 3.2),  this is unlikely to be what Aristotle understood by the term, but given this understanding of induction, Sextus is quick to point out its inadequacy. Those who follow this procedure must survey “all the particulars or some of them.” If they survey only some, the induction will be insecure, since it may be that some of the omitted particulars lack the feature in question. But if they try to survey all, “they will labour at an impossible task, since the particulars are infinite and indefinite” (Sextus PH 2.204). The argument is not strictly sound, since there are cases in which we know (or believe we know) all the instances. An example offered by John Buridan (1295–1358) was that of a proposition regarding all the (six) planets (SD 6.1.5). But the question would remain a live one for any would-be empiricist.