physics

Primary–secondary quality distinction

Primary–secondary quality distinction is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Primary–secondary quality distinction rather than just read about it. In short: 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.

Key takeaways

Primary–secondary quality distinction belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Primary–secondary quality distinction to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Primary–secondary quality distinction from memory before moving on to harder problems.

Reference excerpt

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, while secondary qualities are thought to be properties that produce sensations in observers, such as color, taste, smell, and sound.

Overview

Primary qualities Primary qualities are defined by Locke as those properties of objects that are independent of any observer, such as solidity, extension, motion, number and figure. They exist in the thing itself, and the thing cannot exist without some iteration of each primary quality. That is, if something does not have some size, some shape, some solidity, some motion or rest, or is neither single nor multiple (some number), then it cannot be a physical object. John Locke wrote that when dividing a divisible object, "solidity, extension, figure, and mobility" and number must continue to exist because the primary qualities are built into the object itself. Another key component of primary qualities is that they create ideas in our minds through experience; they represent the actual object. Because of this, primary qualities such as size, weight, solidity, motion, and so forth can all be measured in some form. Using an apple as an example, the shape and size can actually be measured and produce the idea in our minds of what the object is. Locke defines qualities as the ability of objects to produce ideas in our minds. In the case of primary qualities, they exist inside the actual body/substance and create an idea in our mind that resembles the object.

Secondary qualities 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. According to Locke, secondary qualities use the power of reflection in order to be perceived by our minds. These qualities "would ordinarily be said to be only a power in rather than a quality of the object". They are sensible qualities that produce different ideas in our mind from the actual object. Going back to the example of the aforementioned apple, something such as the redness of the apple does not produce an image of the object itself, but rather the idea of red. Secondary qualities are used to classify similar ideas produced by an object. That is why when we see something "red" it is only "red" in our minds because they produce the same idea as another object. So, going back to the color of the apple, it produces an idea of red, which we classify and identify with other red ideas. Again, secondary qualities do not exist inside the mind; they are simply the powers that allow us to sense a certain object and thus ‘reflect’ and classify similar ideas. According to the theory, primary qualities are measurable aspects of physical reality; secondary qualities are subjective.

History Some ancient Greek philosophers including Leucippus and Democritus are thought to have postulated the distinction, which became more formalised in later years:

"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. Later proponents included Galileo, Descartes and Newton:

"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 only in the 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, Opticks (3rd ed. 1721, original in 1704).

Criticism

Leibniz Gottfried 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."

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Primary–secondary quality distinction

Start with the simplest possible case. Write down what Primary–secondary quality distinction claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Primary–secondary quality distinction before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Primary–secondary quality distinction ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Primary–secondary quality distinction

In research: Primary–secondary quality distinction appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Primary–secondary quality distinction in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Primary–secondary quality distinction is common in secondary-school and first-year university syllabi. It links to neighbouring topics Concepts in epistemology, Concepts in metaphysics, Conceptual distinctions, so understanding it makes those chapters shorter.
In everyday life: Look for Primary–secondary quality distinction outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Primary–secondary quality distinction in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Primary–secondary quality distinction means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Primary–secondary quality distinction out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Primary–secondary quality distinction in simple terms?

Why does Primary–secondary quality distinction matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Primary–secondary quality distinction?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Primary–secondary quality distinction.