Story of Psychology - Morton Hunt [49]
All this seems soundly put together and watertight, but there was one serious leak in the system. It was the ancient philosophic problem concerning sense perception: How can we know that what we sense is a true representation of what exists outside the mind? Locke sees no reason to doubt that we have true knowledge of the world around us. He does say, like Descartes, that God would not mislead us, but his comments have the sound not so much of piety as of common sense:
The infinitely wise contriver of us, and all things about us, hath fitted our senses, faculties, and organs to the conveniences of life, and the business we have to do here. We are able, by our senses, to know and distinguish things; and to examine them so far as to apply them to our uses… Such a knowledge as this, which is suited to our present condition, we want not faculties to attain.37
In two respects, however, his discussion of perception created problems for later psychologists. (Locke did not distinguish between sensation and perception; the differentiation would not be made for nearly two centuries.)
First, he accepted the distinction, as old as Aquinas and maintained by Descartes, Galileo, and Newton, between “primary” qualities and “secondary” qualities of the objects we perceive. Primary qualities are “inseparable” from their objects, no matter how much they may change; they produce in us the simple ideas of solidity, extension, figure, motion or rest, and number. “Take a grain of wheat,” Locke says, “divide it in two parts; each part still has solidity, extension, figure, and mobility.” Secondary qualities, such as color, sound, taste, and smell, do not exist in the objects in the form that we perceive them but are sensations that the object’s primary qualities cause in us. A violet is not violet in the dark; it is violet only when it causes a sensation of that color in us. Or so Locke reasoned.
Second, if our ideas are all derived from our perceptions, we know what we perceive but not the reality underlying them—nor even that any reality exists. Similarly, we never know the substance that is mind; we know only our experiences of our ideas. Reasonable Locke is undaunted:
Sensation convinces us that there are solid, extended substances; and reflection, that there are thinking ones; experience assures us of the existence of such beings; and that the one hath a power to move body by impulse, the other by thought; this we cannot doubt of.38
But this simple reassurance would not convince certain other philosophers and psychologists. They would try, and fail, to find a way to prove either that our knowledge of the world is accurate or that anything exists other than our perceptions.
Locke was vague about the nature of mind. Because of his own belief or perhaps in order not to be heretical, he said it was a substance but insisted that we cannot know it any more than we can know the substance behind the qualities we perceive in objects. In fact, in a celebrated passage of the Essay he gingerly suggests that it is as possible to imagine that mind is matter as that it is a different kind of substance:
We have the ideas of matter and thinking, but possibly shall never be able to know whether any mere material being thinks or no; it being impossible for us, by the contemplation of our own ideas without revelation, to discover whether Omnipotency has not given to some systems of matter, fitly disposed, a power to perceive and think, or else joined and fixed to matter, so disposed, a thinking immaterial substance.39
This infuriated the orthodox, who accused Locke of secretly being a materialist and of endangering all of Christian theology. Locke’s psychology survived their attack, and Christianity survived the Lockean threat.
Locke, justly famous for all the foregoing, is often undeservedly credited with being the prime theorist of associationism. It is true that he coined the