Primary/secondary quality distinction
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The primary/secondary quality distinction is a conceptual distinction in epistemology and metaphysics, concerning the nature of reality. It is most explicitly articulated by John Locke in his Essay concerning Human Understanding, but earlier thinkers such as Galileo and Descartes made similar distinctions.
Primary qualities are thought to be properties of objects that are independent of any observer, such as solidity, extension, motion, number and figure. These characteristics convey facts. They exist in the thing itself, can be determined with certainty, and do not rely on subjective judgments. For example, if a ball is round, no one can reasonably argue that it is a triangle.
Secondary qualities are thought to be properties that produce sensations in observers, such as color, taste, smell, and sound. They can be described as the effect things have on certain people. Knowledge that comes from secondary qualities does not provide objective facts about things.
Primary qualities are measurable aspects of physical reality. Secondary qualities are subjective.
- "By convention there are sweet and bitter, hot and cold, by convention there is color; but in truth there are atoms and the void"
- —Democritus, Fragment 9.
- "I think that tastes, odors, colors, and so on are no more than mere names so far as the object in which we locate them are concerned, and that they reside in consciousness. Hence if the living creature were removed, all these qualities would be wiped away and annihilated"
- —Galileo Galilei, The Assayer (published 1623).
- "For the rays, to speak properly, are not colored. In them there is nothing else than a certain power and disposition to stir up a sensation of this or that color."
- —Isaac Newton, Optics (3rd ed. 1721, original in 1704).
Berkeley's Criticism 
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George Berkeley is a famous critic of the distinction. Berkeley maintains that the ideas created by sensations are all that people can know for sure. As a result, what is perceived as real consists only of ideas in the mind. The crux of his argument is that once an object is stripped of all its secondary qualities, it becomes very problematic to assign any acceptable meaning to the idea that there is some object. Not that we can't picture to ourselves (in our minds) that some object exists apart from any perceiver—we clearly think we can do this—but rather, can we give any content to this idea in any particular case? Suppose—and this is the typical case—that someone says that a particular mind-independent object (meaning, an object free of all secondary qualities) exists at some particular spatio-temporal location (in Newtonian terms, in some particular place and at some particular time). Does this mean anything if one cannot specify any place and time? No, in that case it's still a purely imaginary, empty idea. This is not generally thought to be a problem because realists imagine that they can, in fact, specify a place and time for a 'mind-independent' object. What is overlooked is that they can only specify a place and time in place and time as we experience them. Berkeley doesn't doubt that one can do this, but this is not objective, one has simply related ideas to experiences (the idea of an object to our experiences of space and time). Where are the real space and time, and hence the objectivity? Space and time as we experience them are always piecemeal (even when the piece of space is big, as in some astronomical photos), it is only in imagination that they are total and all-encompassing, which is how we definitely imagine (!) 'real' space and time as being. This is why Berkeley says again and again that the materialist has merely an idea of an unperceived object: because we typically do take our imagining or picturing, as guaranteeing an objective reality to the 'existence' of 'something' we have in no adequate way specified nor given any acceptable meaning to, as if having a compelling image in the mind, one which connects to no specifiable thing external to us, guaranteed an objective existence.
- (Quoted by Sextus Empiricus, Adv. Math. vii 135)
- As reprinted in (Drake, 1957, p. 274)
- Reprinted in (Newton, 1953, ed. Chris Jamieson, p. 100)