Where does knowledge come from?

Let us then suppose the mind to be, as we say, white paper ... How comes it to be furnished? from experience.
- John Locke, An Essay Concerning Human Understanding (1690)

Do we really perceive the things themselves, such as trees and cars and other people, or just their sensory qualities?

All knowledge, Locke said, arises from the five senses. There is no idea in the mind without a corresponding sense impression. If Locke had stopped there, he might have been safe from criticism. But he felt called upon to elaborate.

These sense impressions, he said, are of two kinds: primary qualities, such as weight, shape and number, which inhere in the material substance of things; and secondary qualities, such as color, smell and taste, which depend upon the observer. We can know only the primary and secondary qualities of things. Of the material substance or substratum of things, in which the primary qualities inhere, we can have no real knowledge. It is only a je-ne-sais-quois, an I-know-not-what. But it has to be there, otherwise the primary and secondary qualities would not be qualities of anything.

But here is a problem: if all we can know about is sense impressions, and we have no sense impressions of the so-called "material substratum", then we have no knowledge of it, do we? And what is the difference between something that we cannot, in principle, know anything about, and something which doesn't exist?

Suppose someone tells us that there's a pretty rose garden on the dark side of the moon. I know how I would go about finding this out for sure: hitch a ride on a moon-bound rocket, go the the dark side of the moon, and have a look. There are things that are hard to see but are not in principle impossible to see. Locke's substratum, however, is of this latter kind. It isn't just hard to see, or physically impossible to see, but logically impossible to see. It's necessarily unobservable. For all we can observe are sense impressions, and the substratum is not a sense impression.

But there has to be something that sense data are of, doesn't there? That is, I don't just see sense data floating around free. I see a patch of green against a larger patch of blue, and somehow I know that there is a real, material tree there. But how do I know anything about something I don't actually see? And I don't really see the tree, I only see the sense impression of the tree -- the primary and secondary qualities of the thing. Do we see the things themselves, or just their sense impressions?

Bibliography

Locke, John. Essay Concerning Human Understanding, Dover Publications, Inc., New York, 1894, two volumes.

Locke, John and George Berkeley and David Hume, The Empiricists, Anchor Books, Garden City, N.Y., 1974.