Under the unassuming heading "Other Considerations Concerning Simple Ideas," Locke next introduces one of the most important topics in the entire Essay: the distinction between primary and secondary qualities. Locke tells us that there is a crucial difference between two kinds of simple ideas we receive from sensation. Some of the ideas we receive resemble their causes out in the world, while others do not. The ideas which resemble their causes are the ideas of primary qualities: texture, number, size, shape, motion. The ideas which do not resemble their causes are the ideas of secondary qualities: color, sound, taste, and odor.
The best way to understand the distinction between primary and secondary qualities is in terms of explanation. Whenever you have the sensation of a square book the cause of that sensation is some sort of shape out in the world (though not necessarily squareness, since there may be some optical illusion, because distance, for instance, forcing you to perceive the shape incorrectly), so the explanation for sensation of shape is shape in the external world. Whenever you have a sensation of blue, on the other hand, the cause is not blueness out in the world. The cause is some specific arrangement of the insensible parts of matter. Explanations for secondary qualities refer only to primary qualities.
Locke's argument for this claim is based on his estimation of the "best science available", which he believes is Boyle's Corpuscular Hypothesis. According to the best scientific picture we have of the natural world, Locke argues, all that is out there are colorless, tasteless, soundless, odorless corpuscles of matter. Using only these indivisible bits of matter and their motions, we can explain not only our sensations of primary qualities, but our sensations of secondary qualities as well. Sensations of color, odor, taste, and sound are caused by the primary qualities of arrangements of matter. (Locke refers to these arrangements as the "powers" of objects to cause sensations.) Given that we are able to explain everything we need to explain by positing the existence only of primary qualities, he reasons, we have no reason to think that secondary qualities have any real basis in the world. An argument of this form is often called an "argument from parsimony" and rests on the premise that it is best not to posit the existence of explanatorily superfluous entities.
The rest of Locke's discussion of the primary/secondary quality distinction focuses on making the conclusion seem more plausible. He presents a number of thought experiments designed to bring our intuitions into line with his. First, he describes breaking a piece of wheat up into smaller and smaller pieces. He points out that as small as the wheat becomes we cannot conceive of it without its primary qualities (presumably since the very idea of a body without shape or size is incoherent) whereas we can conceive of the wheat without color (presumably because there is nothing literally incoherent about a body without color, even if it is difficult to imagine one in actuality).
He next considers an almond that is being pounded with a pestle. As it gets broken up into smaller and smaller pieces, the color changes from a pure white to a dirtier hue, and the taste goes from sweet to oily. Yet all that was altered was the texture of the nut. Clearly, he concludes, the secondary qualities depend on the primary qualities.
Finally, he takes the example of a flame. If we put our hand in the flame we have a sensation of pain. If we look at the flame we have a sensation of color. No one would claim that pain is in the flame itself, he points out, so why do we suppose that the color is?
Book I, "Of Innate Notions," is an attack on the theory that human beings are born knowing certain things. This idea can take one of two basic forms. Either the theory can be one about principles (i.e. statements of fact) or it can be one about ideas (the sort of things that we have names for, such as "God," "blue," or "existence"). In the first three chapters of Book I, Locke focuses his attention solely on principles. In the last chapter he turns to ideas.
The main thrust of Locke's attack on innate knowledge can be found in Chapter ii. Here he criticizes the possibility of innate theoretical principles. Locke's argument against innate theoretical principles can be captured in three sentences: If, in fact, there are any innate principles, then everyone would assent to them. There are no principles that everyone assents to. Therefore, there are no innate principles. Locke is very careful to demonstrate that there are no principles to which everyone would assent, providing his proof as a dialectic: the nativist (or believer in the existence of innate principles) asserts his claim in its strongest form (i.e. there are certain theoretical principles to which everyone would assent), to which Locke objects. The nativist then revises his claim to accommodate Locke's objection, Locke objects again, and so on until the nativist position becomes trivial. Throughout, Locke's strategy is to focus on those principles which he views as the best possible candidate for universal consent, namely that whatever is is and nothing can be and not be at the same time.
Locke then moves on (in chapter iii) to the possibility of innate moral knowledge. Here too, he claims, there is no universal consent. No man would consent to even the most obvious moral laws without a great deal of reasoning first. Finally, Locke concludes Book I by considering the possibility of innate ideas. On this point he has several lines of attack. First, he draws our attention to developing children (a tactic to which he will appeal repeatedly throughout the text). He claims that they clearly come into the world devoid of ideas, since they only ever seem to have the ideas of those things they have experienced. Next he turns to the ideas which make up the propositions he was investigating in chapter ii--ideas such as "existence" and "identity"--and argues that these are some of the least likely ideas to be innate. These ideas are so obscure and confusing that often one needs several degrees just to become clear on them; obviously, if children were born with these ideas we would not find them so difficult to grasp. (The point here is: since these ideas are not innate, neither are the propositions that they make up. This is just in case you failed to be convinced of the arguments in chapter ii). Last, he turns to the idea of God, the idea he feels is the likeliest candidate for innateness. This idea, however, is clearly not innate, since many cultures recognize no god.
Because the argument for the claim that there is no universal consent for any theoretical principles is long and arduous and also extremely important historically, it demands some detailed analysis. The best way to understand the argument is by breaking it up into dialogue form, giving both the nativist and Locke chances to speak in turn. The dialogue opens with the nativist's statement of his position in unqualified form: There are certain principles that are universally agreed upon and the only way to explain this is to suppose that these principles are innate. Locke's primary reply is that there are no such principles. Even the principles whatever is is and nothing can be and not be at the same time are not agreed upon by idiots. The nativist then refines his position: Our knowledge of these principles does not start out as explicit and conscious knowledge, rather we have tacit knowledge of the principles in question, and it takes some work to make this tacit knowledge explicit.
Locke's response is to call this position incoherent. It is impossible for something to be in the mind without our being aware of it; to be in the mind, to be mental, is to be conscious. This claim is often referred to as Locke's thesis of the "Transparency of the Mental." It is by no means an incontrovertible claim. There is, first of all, the problem of memory; we are not conscious of memories but they are in the mind. There is also the issue of non-conscious principles, propositions, or bits of knowledge. Even when I am not thinking that two plus two equals four, I am tempted to say that I still know it. (In reply to these objections Locke would most likely argue that in order to get into the mind we had at one time to be conscious of these memories and truths.) It is because of cases like these that many philosophers have been tempted to say that knowledge is dispositional; we know something if and only if we know what to do with it once it comes into awareness.
This is exactly the point that the nativist next makes. It is not really that we have tacit knowledge, he says, but that we have an innate capacity or disposition, an inborn ability to entertain certain ideas and arrive at certain principles. Locke dismisses this position, claiming that the doctrine is empty because it ends up saying that everything we know is innate (since we obviously have the capacity to know everything we come to know). He also points out that it does not really qualify as a theory of innate principles, since it admits that experience is required to trigger any and all knowledge.