3. Appearance and reality.
Locke also shared with Descartes what we can call a representational view of mind. According to this view, all the mind has access to are representations (the official Lockean line is that all ideas are either sensory ideas, or ideas of reflection (introspection on the mind's own operations), whereas Descartes allows other kinds of representational capacity and sources for them).
A feature of representations (ideas) is that they can misrepresent: the fact that one has a representation that purports to exhibit some state of affairs does not at all entail that that state of affairs actually obtains. There are not only perceptual illusions, and hallucinations, but even dreams which seemingly create an entire world or alternate reality that need not in any way reflect what is actually the case. The representational view of mind opens a difficult if not impossible to bridge gap between what one can know (the representational contents or ideas within the mind), and what the world is really like. (Descartes' official line to a first approximation was that God, being perfectly good and hence no deceiver, guaranteed the accuracy of the core of Descartes' representations of the world. It is thus God's benevolence that closes the epistemological gap between mind and world.)
Locke's treatment of the epistemological predicament that his representational theory of mind creates is highly inadequate. There are two related aspects to this inadequacy.
First, Locke stipulates without good argument that our ideas of the primary qualities (extension, etc., see Section 4) of things are accurate, that is, the things without us, the things on the other side of the veil of perception, actually have the primary qualities that we perceive them to have (thoug ...