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Shiavault - a Vault of Shia Islamic Books A Guide To Locke's Essay Primary and Secondary Qualities Although Locke assumed that simple ideas of sensation are invariably produced in our minds by the influence of external objects on our organs of sense, he did not suppose this causal process to be straightforwardly representational: there need not always be something in the object that corresponds directly with the idea it produces in us.

In some cases, for instance, it is the absence, rather than the presence, of some determinate feature in the thing that produces "a real positive Idea in the Understanding." [Essay II viii 1-2] Even on a causal theory of perception, privation is a change in the sensory organ, and therefore fully perceptible. Thus, our ideas of cold, darkness, and (perhaps) rest are produced by the absence of heat, light, and motion in things.

Despite the systematic ambiguity of our usual vocabulary, Locke maintained, we should always distinguish between ideas themselves, the immediate objects of thought as entertained by the mind, and qualities, the causal powers by means of which external objects produce those ideas in us.

[Essay II viii 7-8] An examination of the causal processes involved in formation of our sensory ideas thusly led Locke to propose an important distinction among the qualities of bodies: The primary qualities of any body are features Locke took to be inseparable from it, both in the sense that they are the kinds of features we experience to be aspects of every body and in the sense that we suppose that they would remain present even if the body were divided into imperceptible parts.

[Essay II viii 9] Our ideas of the primary qualities include those of solidity, extension, figure, motion, rest, and number. Notice that all of these ideas are perceived by touch and that all except solidity are perceivable by sight as well.

Since bodies operate only on mechanistic principles, our perception of such ideas can only result from a direct contact between our organs of sense and the object itself-or, in the case of vision at a distance, an imperceptible particle from the object itself. [Essay II viii 12] Such qualities therefore exist as features of the body itself, independently of their perception.

Secondary qualities, on the other hand, Locke held to be nothing in the object itself except a causal power to produce ideas of a certain sort-a color, a taste, or a sound, for example-in us.