The last two Lectures have laid out the mechanical, Cartesian analysis of the problems of perception. Every flavour of this analysis, right up to an including modern representational accounts, rests on 'loans of intelligence', in which important work that's required to make the system work is simply given to the system. For example, Descartes proposes a set of cognitive axioms to ground the inferential processes required to implement knowledge, and makes these innate (specifically, given by God). This won't do - if we want to develop a naturalised theory of perception, we need an account of where every part of the system comes from.
The Cartesian programme is a form of rationalism, roughly the claim that knowledge is based in reason rather than experience. This chapter reviews the opposing camp of empiricism, roughly the claim that knowledge is based in experience rather than reason. Turvey reviews the approaches of Locke, Berkeley, and Hume, with respect to how they assert a mind can come to know about the world via experience. They end up running into very similar problems as Descartes et al, because of their continued commitment to the mechanistic hypothesis. We are going to have to reject both of these as we progress.