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.