Now that Locke feels he has demonstrated where knowledge does not come from (i.e. innate principles or ideas), he sets out to show where it does, in fact, come from. This project will consume the rest of the Essay. The picture, on its surface, is exceedingly simple. Knowledge is built up from ideas (the operation by which this occurs is discussed in Book IV). Ideas come in two basic types: simple and complex. Complex ideas are built from simple ideas. All knowledge, therefore, traces back to simple ideas, and simple ideas come exclusively through experience. Book II chapters i-vii are all about the origin and nature of these simple ideas. There are only two ways that a simple idea can find its way into a human mind: through sensation, or by reflection. In sensation the mind turns outward to the world and receives ideas through the faculties of sight, hearing, touch, smell, and taste. In reflection the mind turns toward its own operations, receiving such as ideas as "thinking," "willing," "believing," "doubting." In either case, the process is completely passive. Locke breaks simple ideas down into four categories, each of which receives its own chapter. Chapter iii discusses the ideas we receive from a single sense, such as from sight or touch. The idea of blue and of the sound of a trumpet would be examples of ideas from this category. The idea of solidity, which receives its own chapter (iv), would be another. Chapter v looks at those ideas that get into the mind through more than one sense. Shape and size, for instance, are ideas that arise both from our sense of sight and from our sense of touch. Ideas which come into the mind through reflection are the topic of chapter vi, and chapter vii focuses on those ideas which are the product of both sensation and reflection. As examples of this last type of idea, Locke uses the ideas of unity, existence, pleasure, pain, and substance.
In introducing the notion of simple ideas, Locke claims that we can break all of our experiences down into their fundamental parts. If we see a cat, for instance, we can break that sensation down into blackness, softness, shininess, a certain size, a certain shape, etc. Fundamental bits, those that are "uncompounded, without parts," and cannot be broken down any further, are the simple ideas. On first blush, this definition of simple ideas seems plausible. Certainly, our experiences of the world can be analyzed down into their component parts. However, a little prodding leaves the definition looking less tenable.
Take a solid blue wall, for instance. Surely looking at this wall would yield a single simple idea. It is a prime example of something uniform and uncompounded. Consider, however, the shadows that would inevitably be cast across the wall, as well as the other minute variations in shade that would inevitably be present. Now it is not so clear whether the wall yields a single simple idea or many. One has to wonder whether there really can be an end to this analysis of experience down into component parts, whether there are any fundamental parts that cannot be broken down any further.
As an even more unsettling example, take the taste of wine. To people with unsophisticated palates, this is an uncompounded idea, but other people sense many components in a single sip of good wine. To them, this idea is complex. Locke certainly would not want simplicity to be relative, though. He wants the same experiences to give rise to the same simple ideas in everyone. This criterion for simplicity, then, seems to fail.
Luckily, Locke also puts forward two other candidates as criteria for simplicity, both of which seem more plausible than the first. One criterion is definitional: A simple idea is one that cannot be defined. For example, though we all know what blue looks, no one could give a definition of it, so it qualifies as a simple idea. This seems to hold very well in the case of colors, sounds, tastes, pain, thought, etc.--that is, everything that involves phenomenal experience. It is not as obviously true in the case of existence, unity, solidity and the like.
Locke ventures his last criterion for simplicity much later in the book. In Book III, chapter iv, section 11, Locke claims that simple ideas are those that cannot conceivably get into the mind in any way other than by experience. (In other words, there is no way dream them up or to derive them from someone else's description.) To illustrate, he tells the story of a man who has never eaten a pineapple but wants to know what one tastes like. No matter how much this man reads about the taste of a pineapple, or has a friend describe the sensation in all of its detail, this man will never know what a pineapple tastes like until he eats one. (Contrast this to a man who wants to know what a horse is. Even if he has never seen a horse, he can get an excellent idea of one by reading about them.) Again, though, this criterion seems more applicable to ideas of phenomenal experience than to ideas that do not involve phenomenal experience. Is it really so impossible to get an idea of unity without directly experiencing unity?
Given that the latter two criteria work very well for phenomenal ideas, and are more dubious when it comes to others, perhaps the conclusion to draw is not that there is no such category as simple ideas but merely that non-phenomenal ideas do not have a place in this category. This latter conclusion, unlike the former, would do little damage to Locke's overall theory. Of course, it is also possible that further analysis would reveal that the latter two criteria do, in fact, apply adequately to non-phenomenal ideas.