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 spherical, no one can reasonably argue that it is triangular.
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).
- "[I]t must certainly be concluded regarding those things which, in external objects, we call by the names of light, color, odor, taste, sound, heat, cold, and of other tactile qualities, [...]; that we are not aware of their being anything other than various arrangements of the size, figure, and motions of the parts of these objects which make it possible for our nerves to move in various ways, and to excite in our soul all the various feelings which they produce there."
- —René Descartes, Principles of Philosophy (published 1644/1647).
- "For the rays, to speak properly, are not coloured. In them there is nothing else than a certain power and disposition to stir up a sensation of this or that colour."
- —Isaac Newton, Optics (3rd ed. 1721, original in 1704).
Leibniz was an early critic of the distinction, writing in his 1686 Discourse on Metaphysics that "[i]t is even possible to demonstrate that the ideas of size, figure and motion are not so distinctive as is imagined, and that they stand for something imaginary relative to our perceptions as do, although to a greater extent, the ideas of color, heat, and the other similar qualities in regard to which we may doubt whether they are actually to be found in the nature of the things outside of us."
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George Berkeley wrote his famous critique of this distinction in his book Three Dialogues between Hylas and Philonous. Berkeley maintained 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 Berkeley's 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 one cannot picture to oneself (in one's mind) that some object could exist apart from any perceiver — one clearly can do this — but rather, that one cannot give any content to this idea. Suppose 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). Now, none of this particularly means anything if one cannot specify a place and time. 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 did not doubt that one can do this, but that it is objective. One has simply related ideas to experiences (the idea of an object to our experiences of space and time). In this case there is no space and time, and therefore no 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 argued that the materialist has merely an idea of an unperceived object: because people typically do take our imagining or picturing, as guaranteeing an objective reality to the 'existence' of 'something'. In no adequate way has it been specified nor given any acceptable meaning. As such Berkeley comes to his conclusion that having a compelling image in the mind, one which connects to no specifiable thing external to us, does not guarantee an objective existence.
Kant, in his Prolegomena to Any Future Metaphysics That Will Be Able to Present Itself as a Science, claimed that primary, as well as secondary, qualities are subjective. They are both mere appearances that are located in the mind of a knowing observer. In § 13, Remark II, he wrote: "Long before Locke's time, but assuredly since him, it has been generally assumed and granted without detriment to the actual existence of external things, that many of their predicates may be said to belong not to the things in themselves, but to their appearances, and to have no proper existence outside our representation. Heat, color, and taste, for instance, are of this kind. Now, if I go farther, and for weighty reasons rank as mere appearances the remaining qualities of bodies also, which are called primary, such as extension, place, and in general space, with all that which belongs to it (impenetrability or materiality, space, etc.)—no one in the least can adduce the reason of its being inadmissible."
- Quoted by Sextus Empiricus, Adversus Mathematicos vii 135
- As reprinted in (Drake, 1957, p. 274)
- Descartes, René. Principles of Philosophy. 1644/1647. Trans. Valentine Rodger Miller and Reese P. Miller. D. Reidel Publishing Company, 1984. Page 282.
- Reprinted in (Newton, 1953, ed. Chris Jamieson, p. 100)
- Leibniz, G. W. Discourse on Metaphysics, XII. 1686. Trans. George R. Montgomery. The Open Court Publishing Company, 1902.
- Cfr. Georges Dicker, Berkeley's Idealism: A Critical Examination, Oxford: Oxford University Press, 2011, Part III, paragraph 8: "Berkeley's Attack on the Theory of Primary and Secondary Qualities"