Locke opens the Essay with an attack on the notion of innate knowledge. He is particularly keen on demolishing the nativist position because it had recently gained renewed currency among intellectual circles, partially in response to Rene Descartes' philosophy. Descartes believed that inborn in our minds are certain mathematical ideas (such as the ideas of geometrical shapes), metaphysical ideas (such as the idea of God and of essences), and eternal truths (such as the truth that something cannot come from nothing).
Locke could not have disagreed more, and he spends the entire first book showing us why. He begins by attacking the possibility of innate principles, such as the principle whatever is is. He then moves on to attack the possibility of innate ideas, such as the idea of God and of infinity. Locke only wages this second attack in order to cover all of his bases. The meat of the argument against innate knowledge rests on an argument against innate principles, since only principles (statements of fact), and not ideas (which are the building blocks of these statements of fact, the sort of things that have names, such as "God," "Man," "blue," "existence"), can properly be called "knowledge." I can know (conceivably) that God exists, I cannot know that "God."
The structure of the argument against innate principles is very simple and can be summed up in three sentences. (1) If, in fact, there are any innate principles, then everyone would assent to them. (2) But there are no principles to which everyone assents. (3) Therefore, there are no innate principles. Locke, however, takes a long time making this simple argument because he is meticulous in establishing that there are no principles to which everyone would assent. His proof of this claim takes the form of a dialectic. He formulates a strong nativist position, objects to it, revises the nativist position, objects, and so on until the position left to the nativist is so weak as to be utterly trivial.
As an empiricist, Lock believes that all of our knowledge comes from experience. He further holds that all of our knowledge is built from ideas (think of ideas as little building blocks and knowledge as the structures we create out of them). Taking these two commitments together, he concludes that all of knowledge can be accounted for by accounting for the origin of our ideas. Therefore, Book II, which is all about Locke's theory of ideas, is perhaps the most important part of the Essay.
According to Locke there are two and only two sources for all the ideas we have. The first is sensation, and the second is reflection. In sensation, much as the name suggests, we simply turn our senses toward the world and passively receive information in the form of sights, sounds, smells, and touch. In this way, we receive such ideas as "blue," "sweet," and "loud." In reflection, on the other hand, we turn our mind on itself, and, again passively, receive such ideas as "thought," "belief," "doubt," and "will."
Perhaps the most important issue regarding Locke's theory of ideas is the question of what role an idea is supposed to play in the act of perception. According to the way most people understand Locke, the idea is actually the object of perception. A tree in the external world causes an idea, and this idea, not the tree itself, is what I perceive. This might seem very strange; it is natural to assume that when I have a perception of a tree the object of my perception is the tree. Nonetheless, the majority of philosophers have taken Locke to be saying just this, and there is much evidence to support them. This view of ideas, called the veil of perception because it posits a veil of ideas between us and the world, is still held by many contemporary philosophers of mind.
In Book II Locke distinguishes two very different relations that can hold between an idea and a quality out on the world. Our ideas of primary qualities (size, shape, and motion) resemble the qualities actually in the world; there really is such a thing as shape, size, and motion in the objects we perceive. Our ideas of secondary qualities (color, smell, taste, and sound) do not resemble any qualities in the world. In actual objects there is only size, shape, and motion, and the arrangement of invisible corpuscles somehow causes in us the sensation of such things as color, taste, and smell.
The most accurate way of stating this distinction is in terms of explanation. In order to explain why a piece of wood looks square to me (even if the wood is in fact trapezoidal, and the appearance of squareness is merely an optical illusion), I must refer to shape. An explanation would go something like this: "The wood is shaped like a trapezoid, but because of where I am standing the angles appear so and so." Shape in the external world is always the cause of my sensation of shape, even if the shape out in the world is not exactly the shape I perceive it to be. On the other hand, color in the external world is never the cause of my sensation of color. The size, shape, and motion of insensible particles cause the sensation of color. In explaining why a flower looks blue, there is no reference to blueness out in the world, only to the size, shape, and motion of pieces of matter.
Locke's primary argument for this claim rests on what he calls the "best science of the day": Boyle's Corpuscular Hypothesis, in which all events and states of the natural world can be explained in terms of the motion of tiny indivisible particles of matter called corpuscles. Given this view of the world, all of our sensation can be explained in terms of size, shape, and motion. There is therefore no reason, Locke claims, to assume that there is anything but these qualities in the external world and so we should not make such an assumption. An argument like this one, which rests on the claim that there is simply no need to posit something (rather than on any conclusive proof that the something in question does not exist), is often referred to as an *argument from parsimony*.
Locke's primary concern in Book III is clearing up abuses in language. He thinks that these abuses pose a threat to natural philosophy by ensuring that obscure terms such as "essence" continue to get bandied about and taken seriously, despite the fact that they are utterly incoherent and meaningless as they are currently used. Locke feels that this stubborn adherence to incoherent terms is hindering the acceptance of real scientific progress.
In order to eradicate the abuses of language, Locke first develops a general theory of how our words get their meaning. Then he breaks down types of words, category by category, and shows how we should and should not be using such kinds of words.
Words, Locke claims, refer to ideas. If there is not a clear idea to which our word refers, we should not use that word. In addition, we must take caution to ensure that the ideas to which we refer our words are similar to the ideas to which others refer the same words. Defining our terms and sticking to strict policies of usage are important means by which we can ensure that language does not lead us astray.
Scholastics spoke about essences as those properties which make things the sort of things that they are. Essences, for them, were an obscure and complex matter. Locke attempts to show in Book III that our abstract general ideas are what really do this work of sorting particular things into classes. Essences, which caused so much consternation for so long, are nothing but general ideas of the mind.
These general ideas are formed by gathering together ideas of particular things and attending to the similarities among these things. For instance, to form the idea of "cat" I would take my ideas of Frisky, Snowball, Felix, and Garfield and abstract out the tail, the furriness, the size, the shape, the meow etc. I would take all of these similar observable properties and forge them into a new idea, the idea of "cat." This new general idea is what determines what in the world counts as a cat. If an animal fits my idea, then it is a cat. If it does not, then it is not.
This method of individuating sorts makes categories entirely conventional rather than natural. Locke believes that there are no natural kinds in the external world. Instead, there is a continuum of nature, and we impose boundaries among chunks of this continuum for our own purposes.
Locke calls the essence that is responsible for sorting individuals into classes the nominal essence. The nominal essence, again, is just the abstract general idea, which is just a collection of observable properties. In addition to the nominal essence, objects also have a real essence. The real essence of a thing is based in its internal constitution. The real essence is that part of the internal constitution that gives rise to the observable qualities that make up the nominal essence.
Though a real essence has a basis in the world, rather than just in our minds, Locke argues that it cannot be used to sort things into natural kinds. This is so because, first of all, we cannot observe the internal constitution of things. In addition, even if we could observe the internal constitution of things (say, with a powerful microscope) real essences still could not help us sort things into classes. The real essence is itself determined by the nominal essence. Internal constitutions give rise to a myriad of observable properties. It is only the parts of the internal constitution that gives rise to those properties included in the nominal essence that become a part of the real essence. What counts as the real essence, then, is based entirely on how we carve up nominal essences.
The entire Essay builds up to Locke's theory of knowledge. The upshot of this theory is that knowledge is possible but limited. He is arguing here primarily against the rationalists, who believed that our capacity to know is virtually limitless, and the skeptics, who believed that we are incapable of knowing anything at all.
Locke gives a strict definition of knowledge, whereby one can only be said to know something when one sees why it is necessarily so. That is, knowledge depends on the perception of a necessary connection. This is much the same definition of knowledge that Descartes and the other rationalists used, but in Locke's empiricist hands, it has very different consequences for the human capacity to know.
According to the Cartesian Rationalists, the entire world is made up of a web of necessary connections that the mind, with its use of reason, can potentially unravel. Locke, however, does not believe either of these claims. First of all, he denies that the mind is capable of grasping every necessary connection there is because he thinks that our only source of information is experience and experience does not reveal all the necessary connections to us, as these lie in the unobservable underlying microstructures of object. In addition, he does not believe that there is a necessary connection behind every question; there is no necessary connection linking the unobservable microstructures to the secondary qualities we experience. There is no reason, for instance, why the microstructure that currently gives rise to our sensation of yellow had to give rise to our sensation of yellow, rather than our sensation of blue. The connection between the microstructure and the sensation it produces in us is based entirely on the arbitrary decision of God.
Since all of our access to the natural world is founded on observable properties and we cannot grasp the necessary connections that account for these (or, in the case of secondary qualities, not account for them), Locke concludes that we cannot have any knowledge regarding the nature of things. This is tantamount to saying that science (other than the purely mathematical sciences and the science of morality) can never result in knowledge.