There is much more falsehood and error among men than truth and knowledge 4.15.6
In book four of the Essay Concerning Human Understanding, Locke attempts to describe the limits of knowledge, and to draw distinctions between certain knowledge, probable knowledge, truth and opinion. What is most interesting about this part of the Essay, is that Locke sets out a process based philosophy, not a content based one. Which means, that he is more interested in the functioning of the mind, of mental processes, of how the mind forms and uses ideas, rather than in the content of those ideas, whether they are moral ideas, political, religious and so on. Locke's philosophy of mind is almost mechanistic, psychological. It cannot be proved except by the inward 'listening' of the reader to the processes of his or her own mind. To be sure, in the Essay Locke does mention morality and religion, but these are as illustrations of the mental processes he is examining, not as philosophical concerns in themselves.
The basic foundation that Locke builds on is that the mind can only have knowledge of its own ideas. Remember, that for Locke, the mind is a discrete entity, non-physical, enclosed, isolated from the world, and aware only of its own processes. When we think about a tree, we are not thinking about a tree in the world, but about the idea of a tree in our mind. The idea of the tree arrives in our mind by means of the senses, but the mind has no awareness of the tree as a tree, only of the idea of the tree as the senses cause this idea to arise in the mind.
Basically, knowledge consists of the perception of agreement or disagreement between two ideas, and is limited to this only. All forms of knowing come down to this perception of agreement or disagreement. Nothing else is possible.
All our knowledge consisting as I have said in the view the mind has of its own ideas, which is the utmost light and greatest certainty we, with our faculties, and in our way of knowledge, are capable of... 4.2.1
The mechanics of knowing
Locke begins by describing how the mind perceives ideas with a four step process. First, the mind perceives the identity or diversity of an idea, which is to say, the mind is aware that the idea is this idea, and not another idea. Second, the mind then perceives that this idea exists in relation to another idea. Then, the mind perceives the necessary coexistence of these two ideas, or not. Finally, the mind considers the connection between the idea and reality, or what it knows of reality through the other ideas it has of reality, or of the likely connection to reality. Having looked at the characteristic of an idea, Locke now turns to how the mind forms knowledge through the placing of ideas together.
The mind perceives the agreement or disagreement between two ideas in two ways. First, the immediate awareness that two ideas agree or disagree. This is the strongest kind of knowledge, and Locke calls it intuitive knowledge. Where the immediate agreement or disagreement of two ideas is not possible, the mind then tries to make these two ideas agree or disagree by the intervention of a third idea, which links the two first ideas, and which has an intuitive relationship with each of the two ideas. Thus, a kind of chain of connected ideas is built up, in which ideas intuitively connect to the next idea in the chain, but not to next idea after that. This kind of knowledge is demonstrative knowledge. All forms of knowledge reduce down to these two basic operations.
There are two kinds of knowledge, actual knowledge, in which the mind is aware of the agreement or disagreement of two ideas as they happen, or is aware of the process of chaining as it happens, or consciously sets the process in action; and habitual knowledge, in which the intervening steps in the chaining of ideas have been forgotten, or are taken as having occurred. The second, habitual knowledge, is of course the most common, as it is undesirable, unnecessary, and impracticable for the mind to constantly form chains between all the ideas it perceives all the time. We do not need to go through all the steps in the chain to know that the sum of the areas of the two squares on the two sides of a right angle triangle equals the area of the square on the hypotenuse. We take the intervening steps as having been demonstrated before.
Forms of knowledge
In Locke's time, knowledge - learning- was enshrined by scholars, by the academy, in the form of maxims, axiom, propositions and syllogisms. Locke attacks all of these as the basis of knowledge, on the grounds that they are generalisations. Knowledge for Locke does not proceed top-down from generalisations about the world to particular examples, but proceeds bottom-up, from a perception of particulars, to a combining and sorting of those particulars, to a naming of them, and finally to a generalisation of them into maxims and axioms. Locke thereby disagrees with the concept of innate ideas - knowledge as top-down maxims- and seeks to establish a basis for his own epistemology: - knowledge as bottom up empiricism. For in particulars our knowledge begins, and so spreads itself by degrees to generals. 4.7.11
Most surprisingly, perhaps, is Locke's dismissal of the syllogism as a means for arriving at knowledge. Regarded as one of the foundation stones of reasoning since Aristotle and the Stoics. Locke points out that people can reason very well who have never seen or heard a syllogism, and that the syllogism throws the mind into perplexity. The usefulness of a syllogism is limited to one part of reasoning only, that part which shows the connexion of proofs - that is to say, the intervening steps in demonstrative knowledge. Syllogisms for Locke are useful only in what he calls 'fencing' with knowledge, in debate and winning arguments; but as a means for improving knowledge, they are not helpful.
What I find most interesting about Locke's epistemology is that he takes knowledge out of the hands of those who would restrict knowledge for their own use - those who lay down the maxims and axioms of various disciplines (specifically, out of the hands of the Church) - and gives it to anyone who is capable of an internal 'listening' to their own mental processes.
Locke's language is one of the marvels of expository English. He proceeds slowly carefully, pedantically, building up his ideas from small units, laying out his taxonomies and numbering them. Each positive idea is qualified and complemented by its negative so that no misunderstanding may occur. He knows the secret of good pedagogy, which is frequent repetition with minor variations to make the ideas stick in the mind. When he is difficult, it's because his ideas are difficult, not because his language is unclear. His use of images and adjectives is always felicitous: he describes the mind manacled in a chain of syllogisms 4.17.5 and the flash of intuition as evident lustre and full assurance 4.2.6 and indulges in cutting sarcastic asides on those who seek to obfuscate and control knowledge: the increase brought into the stock of real knowledge has been very little in proportion to the schools, disputes and writing the world has been filled with... 4.3.30
The way to improve our knowledge, is not, I'm sure, blindly and with implicit faith to receive and swallow principles, but is, I think, to get and fix in our minds clear distinct and complete ideas as far as they are to be had, and annex to them proper and constant names 4.12.6