The Sceptics (with a capital "S," that is the classical Sceptics) argued that knowledge was impossible. This is the opposite of the Protagorean relativist position that one cannot have a false belief, since judgements about the world are entirely internal to the judger. Basically the Sceptics are realists about the truth (an interesting distinction between classical Scepticism and Cartesian scepticism, which is motivated by the same kind of internalist view of knowledge held by Protagoras). Thus the problem is that perception requires judgement, and judgement is corrigible. Note the distinction between sensation (a prereflective experience of leafy, green, big, rustling, etc.) and perception ("I see a tree."). The big fish here is the idea that perception (and by extension thinking in general) involves "bringing sensations under concepts," as this goes to the heart of Cartesian, Kantian, Fodorian, and other versions of nativism, the view that mental representation requires some a priori conceptual structure, and related claims that, for example, only beings with language are capable of genuine perception (e.g. Chomsky) (and if you've read other parts of this blog you know that I think that this sort of view is profoundly mistaken).
Aristotle introduces the notion of nous as an antidote to Scepticism. Some genuine examples of knowledge, Aristotle argues, are not products of sensation plus judgement, but rather are produced by sensation ("experience" is more intuitive and brings out the empiricist bent of Aristotle's thinking) directly. This circumvents the Sceptical objection that we have no "metalogic" with which to check the judgements involved in belief formation: noetic beliefs are formed prereflectively. This is surely better than the Kantian approach: a nativist has to argue that animals and babies don't know anything if they can't bring their experience under concepts, which seems to involve a covert equivocation on the meaning of the word "knows" (since animals and babies know lots of things when we're not doing philosophy!), and if nativism means anything significant it has to be that conceptual structure cannot be explained with natural history and learning. Indeed the nativists tend to admit this and even make a virtue of it, Descartes and Kant holding that conceptual structure distinguishes rational minds from physical phenomena, Fodor admitting that to accept his view we must accept that cavemen had the concept of airplanes, and Chomsky insisting for many years that evolutionary explanations could never explain the emergence of language (even though evolutionary approaches straightforwardly show how "innate" structures can be accounted for naturalistically). Final pregnant thought (brainchild or wind egg, you decide): noetic approaches to perception may be a useful component of eliminativist approaches to mental representation.