For I imagine anyone would easily grant, that it would be impertinent to suppose, the ideas of colors innate in a creature, to whom God had given sight, and a power to receive them by the eyes from external objects: and no less unreasonable would it be to attribute several truths, to the impressions of nature, and innate characters, when we may observe in ourselves faculties, fit to attain as easy and certain knowledge of them, as if they were originally imprinted on the mind.
This statement neatly sums up Locke's entire purpose in the lengthy Book II, though it is made at the very start of Book I. In Book I, immediately following this quotation, he attempts to demolish the position of *innate ideas* and principles on their own terms. In Book II he turns to the more important task of demonstrating how our faculties are, in fact, able to cull from experience all the ideas that fill our head. If he can really account for all of our ideas by tracing them to experience, he will undermine the need for innate ideas and strengthen the empiricist position considerably.
Let us suppose the mind to be, as we say, white paper, void of all characters.
This is the well-known "tabula rasa" passage. It is probably the most famous statement of the empiricist position. By calling the mind a blank sheet of paper, Locke means to claim that the mind at birth contains no ideas. Experience must then "write" on the mind by furnishing it with ideas.
Pound an almond and the clear white color will be altered to a dirty one and the sweet taste into an oily one. What real alteration can the beating of a pestle make in any body, but an alteration of the texture of it?
This is one of Locke's most famous thought experiments. It is meant to demonstrate that secondary qualities, as we perceive them, are not really in objects themselves. Instead, all that is in objects is primary qualities, such as size, shape, and motion. The secondary qualities that we experience are simply caused by various arrangements of primary qualities. Though an almond tastes sweet to us and looks white to us, there is no taste or color in the almond. His proof? When we alter the taste and color, all we are really doing is pounding the almond into smaller pieces. We are altering the primary qualities of the almond, and yet the change also affects the secondary qualities. Secondary qualities, therefore, exist in objects only as arrangements of primary qualities.
Thus we may conceive how words, which were by nature so well adapted to that purpose, came to be made use of by men, as the signs of their ideas.
This is a concise statement of Locke's theory of meaning. Words, in his view, do not refer to things in the external world, but to the ideas in our head. When you say "dog," for instance, you are not really referring to any dog out in the world, you are referring to an idea you have in your mind of something furry, four legged, loyal, and panting. Even if you say "Lassie" you are not referring directly to that creature you see running around the television screen. You are referring to the idea you have of the dog running around the television screen, in this case the idea that is, or once was, your sensation of that particular dog.
Our knowledge in all these enquiries reaches very little farther than our experience.
This quotation expresses Locke's estimation of the capacity for human knowledge. The "enquiries" to which he is referring include all of natural science. Locke's definition of knowledge is extremely strict; he believes one can only be said to know when one perceives a necessary connection. That is to say, I can only know that A caused B if I can deduce from A that B had to happen. To put it another way, looking simply at A I can predict with absolute certainty that B will happen. Short of this sort of understanding, all one is left with is opinion or belief.