After his discussion of the origin of simple ideas, with the lengthy detour on the topic of primary and secondary qualities, Locke discusses the operations utilized by the mind in order to do things with ideas. In Chapter ix, he discusses the faculty of perception, which includes both sensing and reflecting. Chapter x addresses the faculty of retention, which includes contemplation and memory. In Chapter xi, he discusses five other faculties: discerning, which is the process of distinguishing one idea from all others; comparing two ideas to one another; composing an idea from two or more others; enlarging one simple idea into a complex one by repetition; and abstracting certain simple ideas away from an already complex one.
Perception is already familiar to us from previous chapters, and much that Locke tells us here is elaboration on themes we have seen before. He tries to impress upon us once again that that perception is "the first step toward knowledge and the only inlet of all the materials in the mind," (II.ix.15) and also that perception cannot occur without awareness. He also has some new points to add, namely that perception often comes attended by some automatic judgment, and that perception is what separates animals from lower forms of life.
The discussion of judgment's role in perception is well known, primarily because it is in this context that Locke introduces Molineux's Problem. Sometimes when we have a sensation, Locke tells us, the idea received by that sensation is automatically altered by judgment without our being aware of it. For instance, when we look at a blue sphere we see a solid color despite the fact that reflections of light cause minute variations in shade. Given this role that judgment plays in the formation of our ideas from sensation, Locke is extremely concerned that we not overestimate judgment's importance in this process. He, therefore, presents is with Molineux's problem: If a blind man who can distinguish between a sphere and a cube of metal by his sense of touch suddenly regains his eyesight, can he tell the cube from the sphere without touching them? The answer, both Molineux and Locke agree, is "no." The reason is that only experience can give that information; judgment cannot fill in the gaps.
With regard to memory, Locke is concerned mostly to tell us which ideas are best remembered and to name the defects of the faculty. Locke recognizes that, given his doctrine that all mental items must be conscious, there is not much room allowed for memory. For the sake of consistency Locke admits that by "memory" he does not literally mean a place where ideas are stored, but rather, he refers to a power of the mind to revive perceptions it once had.
Discernment, or the ability to tell one idea apart form all others, is the faculty that determines intelligence, according to Locke. The stronger this faculty, the stronger one's powers of reasoning. Discernment makes our ideas clear and determinate, qualities that are revealed as extremely important for knowledge in Book IV. The other four faculties discussed in chapter xi receive greater treatment within the context of discussions about complex ideas.
Locke's theory of perception does not distinguish between sensations and sensory perceptions, two concepts that have become very important to philosophers of mind, and for this reason it can seem a bit unsophisticated to modern readers. When an object in the world impinges on human sense organs and produces a conscious result, we tend to think now that there are two mental items to distinguish: there is the conscious result itself, the cognitive state of mind, which is of the object out in the world (the sensory perception), and then there is the simple act of the perception, something which is not of anything at all. For Locke, however, there is simple one state, the perception, and many critics think that this dangerously simplifies the picture.
Another important point to note about Locke's theory of perception is that it is a mediated theory of perception. That is to say, according to Locke we do not have direct access to the world around us, but rather this access is mediated through our ideas. According to Locke, all that we have direct access to is our own subjective states of mind. This mediating layer of ideas is referred to as a "veil of perception." Some philosophers have tried to resist reading Locke in this way trying to paint alternative pictures of Locke's view in which Locke is a direct realist, meaning that he believes that we directly perceive the world. Whether Locke is a direct realist or a mediated theorist hinges entirely on what Locke considers the true nature of ideas. There are three possible ways to understand what an idea is. Ideas may be transparent windows, meaning that we could perceive right through them to the world. In other words, ideas may merely be conduits to the world that do not block our view of it. Ideas might also be translucent, coloring our experiences but not obscuring our view. Finally, on the standard interpretation, ideas are opaque. All that we perceive are our own ideas, and in order to get to the external world, we must make an inference. There are advantages to both naïve realism and a mediated view of perception. On a mediated view of perception, we do not have to worry if our concepts match our objects of perception. We do, however, have to worry about our knowledge of the external world. If we only have direct access to our own ideas, how can we be sure what the world beyond them is like? For all we know our ideas are being caused by something wholly unlike the world we believe we are perceiving, and we really have no way to verify whether or not this is the case. The best evidence that Locke believes in a mediated view of perception is the effort he exerts in order to show that he can get around this latter problem. Book IV, chapter xi is devoted to the argument that, though our knowledge of the external world is only by inference, it is still so strong and justified an inference that it counts as certain knowledge. If Locke did not believe in a mediated view of perception, it is difficult to explain why he felt that we only arrive at direct access to the world through inference. Turning from perception to memory, we see Locke struggling to remain consistent with his doctrine of the Transparency of the Mental. His compromise, that memory is simply the ability to recall certain ideas to the mind, leaves many questions. First of all, where are these ideas when they are not in the mind? Do they simply disappear? If so, then how are they so readily available to be recalled? If they do not disappear, then Locke once again runs the risk of violating the rule of the Transparency of the Mental. Unfortunately, if Locke was aware of these worries, he did not mention them. We are, therefore, left with little material to use in formulating a satisfactory solution to these puzzles.