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.
It is not at all clear that Locke's response here is adequate to disprove the nativist position. At the very least, there is a nativist position very close to the one stated here that escapes Locke's criticism. This is the position held by the philosophers G.W. Leibniz and Immanuel Kant. According to Leibniz, we have a disposition toward knowing certain things and we can find the basis for this knowledge in ourselves through introspection. He appeals to the distinction between the context of discovery and the context of justification in order to make this claim. While experience may be required to discover truths, it cannot be the basis for our knowing them. We could never arrive at claims about all objects through experience since we never experience anything but a very limited number of objects. This justification must somehow be supplied by the mind, and Leibniz claims that the justification is supplied through innate dispositions toward knowledge. Kant's notion of the categories of thought plays a similar role; the categories act as innate molds into which we form our experience in order to arrive at knowledge.
Neither Kant's nor Leibniz's formulation of the nativist position is empty in the way that Locke claims. In addition, both can be seen as real nativist claims, contrary to what Locke argues. Though experience is necessary to trigger knowledge on these models of the mind, experience is not sufficient for knowledge. Innate categories of thought, in the form of certain innate principles, are needed if human beings are to arrive at any knowledge through experience.
Locke's nativist, however, is unaware that he has not yet lost this particular argument. Rather than pointing out that Locke misunderstood his claim (which, of course, would be impossible, since the nativist is simply Locke playing devil's advocate with himself) the nativist tries once again to reformulate his position. Locke and the nativist continue to wrangle for a few more pages before Locke considers the battle won. Given his crucial mistake in characterizing the nativist claim regarding dispositions, however, he is not really justified in believing that he defeated the nativist. Whether or not he might have been able to think up an objection to the position as formulated by Leibniz and Kant, he failed to do so in the Essay, and so as an argument against innate knowledge, Book I fails.