Two stock characters constantly put in their appearance in Reid's writing: the madman and the skeptic. I shall introduce the madman in the next chapter; for now, it's the skeptic – though the madman will put in a brief appearance as well.

Skeptics come in many types. Reid has his eye on just one – a type that haunts Western philosophy from the seventeenth century onward. Whether it's quite right to call him a skeptic is a good question. Apart from his dissent on certain epistemological issues, he doesn't actually doubt more than the rest of us.

REID'S SKEPTIC

Who is Reid's skeptic? Recall Reid's standard schema for perception: The perceived object evokes in the perceiver a sensation that is a sign of itself; this sensation then evokes a conception of the object and an immediate belief about it, that it exists as something external (or a belief which entails that). In perceiving the sun, the sun evokes in me a sensation that is a sign of itself; and that sensation evokes in me an apprehension of the sun and the immediate belief, about it, that it exists as something in my environment. It's to that immediately formed belief, and the apprehension that it presupposes, that Reid's skeptic directs his attention. What he has on his mind is the fact that sometimes what's immediately evoked by sensory experience, though taken by the perceiver as an apprehension of, and belief about, some external object, is not that, since there's no object that stands in the requisite relation to the sensation for the latter to have been the right sort of sign of it.