 Does this scepticism rule out any kind of general knowledge? The sceptics, of course, are not interested in developing general laws or principles. If they do discuss scientific matters, it is only to show that for any one theory, a contrary but equally persuasive theory can be developed (Sextus PH 1.18). But they do not deny that for practical purposes we can rely on the observed association of particular kinds of events. Of importance here is Sextus’s distinction between two types of sign: the “commemorative” or “recollective” (hypomynēstikon) and the “indicative” (endeiktikon). An indicative sign is thought to signify some reality that has not directly been observed. It does so on account of what Sextus calls “its proper nature and constitution” (Sextus PH 2.101). The idea here seems to be that of a necessary connection between the sign and what it indicates, known by reason rather than by experience. The inference from movement to the possession of a soul, for example, makes use of an indicative sign, for the soul can never be directly observed. In Sextus’s view, inferences of this kind cannot be justified. Sextus is, however, happy to endorse the use of commemorative signs, which are based on nothing more than the past experience of the association of two kinds of phenomena. Smoke, for instance, is a commemorative sign of fire; a scar is a commemorative sign that the bearer has received a wound (Sextus PH 2.102).