PRIMARY AND SECONDARY QUALITIES
Naji Rodes
Early Modern British Philosophy
Revised and Expanded Paper 1
March 24, 2015

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Locke proposes a theory of qualities based on a distinction between qualities that resemble the idea we have of them and those that do not.
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I will begin by interpreting Locke’s theory of primary and secondary qualities, distinguishing between phenomenal and nonphenomenal primary qualities, and showing that it is most sensible to interpret Locke’s claim that bodies do not really have secondary qualities as another way of saying that our ideas of secondary qualities do not resemble the qualities. I will then interpret and defend Strawson’s response
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to Locke as a demonstration that no primary qualities are phenomenal primary qualities, showing that statement to be equivalent to the conclusion that we do not perceive objects the way they really are. I will finally suggest that, if Strawson’s response does indeed require something in Locke’s theory to be altered, that thing is the principle of economy of explanation.
First, I summarize two definitions. By idea, Locke means “Whatsoever the mind perceives in itself, or is the immediate object of perception, thought, or understanding.” 3
This can be understood to mean that, when the ultimate object of thought is not an idea, the thought occurs through an idea that stands in the place of the object ultimately being thought about. By quality, Locke meant “the power to produce any idea in our mind.” 4 But he later makes a concession to the ordinary definition of quality by including in his philosophical definition such powers as that of “fire to produce a new color, or consistency in wax or clay.”
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Thus a quality means either a power to produce an idea in a mind or a power to alter the physical world.
Next I consider Locke’s definition of primary qualities. Initially, he defines a primary quality as a quality that is “utterly inseparable” from the body it is in.
6 Here his inclusion of
1 John Locke, An Essay Concerning Human Understanding , ed. Kenneth P. Winkler (Indianapolis: Hackett
Publishing Company, 1996).
2 Peter Strawson, “Perception and Its Objects,” in Vision and Mind: Selected Readings in the Philosophy of
Perception , ed. Alva Noe and Evan Thompson (Cambridge, MA: MIT Press, 2002), 98-99.
3 Locke, 48.
4 Ibid., 48-49.
5 Ibid., 49.
6 Ibid.

those qualities which “the mind finds inseparable from every particle of matter, though less than
2 to make itself singly perceived by our senses” 7
indicates that primary qualities include that sort of quality which is the power to alter the physical world, even if there is not also a power to produce an idea in a mind. He then gives another definition by stipulating that “the ideas of primary qualities of bodies, are resemblances of them, and their patterns do really exist in the bodies themselves.” 8 Since Locke gives no convincing reasons why all qualities which are inseparable from bodies resemble ideas in the mind or why all qualities which resemble ideas in the mind are inseparable from bodies, I will make use of only the definition that involves resemblance to an idea in the mind.
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Additionally, it will be noted that there might exist some primary qualities which resemble not just any idea we have, but resemble the very same idea that they directly produce in us through the senses. These I will refer to as phenomenal primary qualities because the ideas we would have of them would constitute a direct, sensory grasp of the physical world. We can arrive at ideas that resemble non-phenomenal primary qualities by a process of judgement or inference from the ideas that are directly produced in us by the senses.
In summary, primary qualities are defined as those powers (to produce an idea or to alter the physical world) which resemble an idea we have. Phenomenal primary qualities are those primary qualities which resemble an idea that they produce in us directly by the senses.
Locke defines secondary qualities as “Such qualities , which are in truth nothing in the objects themselves, but powers to produce various sensations in us by their primary qualities
.” 10
Bodies have secondary qualities in the sense that they have a configuration of primary qualities which together have the power to produce an idea in the mind. However, secondary qualities do
7 Ibid.
8 Ibid., 51.
9 It will be noted here that not all qualities of the sort which are a quality in virtue of being a power to alter the physical world will be included in this definition. Bodies may have powers of which we have no idea but which nonetheless alter other bodies. These qualities will remain outside the category of primary qualities until such time as some combination of observation and inference to the best explanation gives us an idea that resembles that quality.
10 Ibid., 49.

not resemble the ideas that they form in us.
11 This is because secondary qualities do not directly
3 produce in our minds an idea of a configuration of primary qualities. However, it is true that we could come to have an idea that resembles a secondary quality by having an idea of the configuration of primary qualities that is the secondary quality. Locke stresses that secondary qualities are not “really in”
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the bodies, with “really in” presumably being equivalent to “in the way our ideas suggest they are in.” It would be absurd to interpret Locke as claiming that bodies do not have secondary qualities in any sense of the term “have,” since that would imply that bodies do not have the power to produce ideas of colors, odors, or temperatures in our minds.
Now I will address Locke’s method of classification of the qualities. He identifies extension, shape, number, position, and motion as the only primary qualities.
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All other qualities, such as temperature, color, taste, sound, smell, and texture, he considers to be secondary qualities.
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He defends his identification of primary qualities on the grounds that these are necessary for a complete account of how the physical world works. Such an identification is not the last word on which qualities are primary; it is subject to the latest empirical investigations and the most efficient explanations of their results. Lock defended his identification of the secondary qualities by citing an experiment that could be done on each quality which supposedly shows that the quality depends too much on the observer or the conditions of observation for it to be primary. For example, he claims that the phenomenon of the same water feeling cold to a warm hand but warm to a cold hand proves that temperature cannot be a primary quality.
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But this method of reasoning overlooks the possibility that there is just some very complicated primary quality that has the sole function of producing the idea of warmth in cold hands and the
11 Ibid., 51. Here resemblance refers to something stronger than the vague similarity we use it to refer to in everyday speech. The way Locke uses resemblance, an idea resembles a quality (or vice-versa) when the idea is an accurate representation of the quality. It may be a fruitful line of inquiry to figure out if there are any ideas that fall somewhere in between the extremes of resemblance and non-resemblance, but I will not explore it here.
12 Ibid.
13 Ibid., 54.
14 Ibid.
15 Ibid., 53.

4 idea of coldness in warm hands. Nonetheless, it is possible to account for temperature, color, taste, sound, smell, and texture by considering them to be complex manifestations of the qualities
Locke identified as primary. Doing so simplifies our assumptions about bodies. The principle of economy of explanation can then be used to show that we ought to consider as secondary all those qualities which our most efficient account of the physical world does not require to be primary.
I will present Strawson as taking Locke’s theory to its natural conclusion before I discuss how that action can be construed as a criticism of Locke. Strawson shows that it follows from
Locke’s theory of primary and secondary qualities “that we do not perceive objects as they really are.”
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He begins by defending the proposition that primary qualities only present themselves to the senses through secondary qualities. He justifies this by citing the basic observation that we can only become aware of the physical world through sensory modes such as vision, touch, audition, and olfaction. These sensory modes can only perceive qualities such as color, brightness, texture, pressure, temperature, sound, and odor, none of which are identified as primary by Locke.
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It is only indirectly through ideas of these secondary qualities that we develop ideas of distinct bodies or the ideas of the primary qualities of bodies, such as the idea of extension, shape, number, position, and motion.
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This means that the primary qualities of bodies cannot be perceived without their secondary qualities also being perceived. Strawson combines
16 Strawson, 98-99. He makes it clear that by perception he means specifically sense-perception. His term
“object” is presumably synonymous with Locke’s term “body.”
17 It is true that this is not logically derivable from the definitions of primary and secondary quality. It is just a matter of fact about our sensory modes that the ideas they directly produce in us by sensation are not the same ideas as the principle of economy of explanation requires us to consider as resembling bodies themselves. If the most efficient way to explain the particular colors, temperatures, odors, etc. that we perceive involved attributing qualities resembling our ideas of color, temperature, odor, etc. to bodies, then we would perceive objects as they really. But it is not likely that such an explanation could ever be more efficient than either corpuscularianism or more recent scientific accounts in which color, temperature, odor, etc. are secondary.
18 You can satisfy yourself of this statement by a few simple experiments. When you are watching a projectile in motion, can your vision determine the motion of the projectile without employing a judgement based on the changing pattern of color and brightness? If that projectile strikes you, can you feel its solidity independently of the pressure that it exerts on you? If you then pick it up and grasp it in your palm, can you feel its shape or extension independently of the pressure on you figures in combination with the proprioception that you use to infer to what extent the object permits your hand to close around it? Can you see its shape or distinguish it from the background independently of the pattern of brightness and color you perceive?

5 this with the part of Locke’s theory that says secondary qualities are not really in bodies and hence that any act of perception involving the perception of secondary qualities does not count as perceiving bodies the way they really are. We must perceive the primary qualities of a body in order to perceive the body the ways it really is because the primary qualities are part of the way the body really is. But we do not perceive the primary qualities; we only infer them from the secondary qualities, which are not really in bodies. Therefore, we cannot perceive bodies the way they really are.
Strawson’s key premise that we only develop ideas of distinct bodies and primary qualities through secondary qualities can be rephrased in terms of resemblance. Recall the distinction I’ve made between phenomenal primary qualities, which directly through senseperception produce in the mind ideas resembling themselves, and non-phenomenal primary qualities, which are only primary qualities because intellectual activity such as inference to the best explanation combines with the non-resembling ideas we have from sensation to create complex ideas in the mind that resemble the primary qualities. Strawson’s argument then becomes that there are no phenomenal primary qualities.
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As I’ve discussed, the evidence for this is a comparison between the set of ideas we get directly by sensory modes and the set of ideas our best explanation of the physical world requires that qualities resemble. The comparison reveals little or no overlap. Therefore, none of the ideas we get directly through sense-perception resemble qualities in bodies.
This conclusion is synonymous with Strawson’s conclusion, as it is stated by him, that we cannot perceive bodies the way they really are. The word “really” first enters Strawson’s argument when he invokes Locke’s principle that primary qualities are really in bodies but secondary qualities are not. I have shown that it is most sensible to interpret this as another way
19 It is true that the statement that there are no primary qualities which directly produce an idea of themselves in our minds is more specific than the statement that there are no ideas of primary qualities directly produced
(by any quality) in the mind. Only the more specific is really needed here, though both can be justified.

of saying that primary qualities resemble an idea in us but that secondary qualities do not.
“Really” is then equivalent to “in a way that resembles our ideas.” Applying this interpretation,
6 the result that we cannot perceive bodies as they really are becomes the result that we cannot have an idea directly through sense-perception that resembles a quality in a body. These are two terminologically distinct methods of using synonymous premises to arrive at synonymous conclusions. But one must be careful not to misinterpret Strawson’s conclusion. It is still perfectly possible to have an idea that resembles a quality in a body; it is only that such an idea must come from a combination of sense-perception and a mental act of judgement, an inference to the best explanation.
Strawson’s argument can be described as a criticism of Locke. But it does not draw attention to any internal inconsistency, question the definitional distinction between primary and secondary qualities, or question whether Locke identified the correct qualities as primary and the correct qualities as secondary. Rather, Strawson’s argument is a criticism because it shows that a conclusion follows from Locke’s theory which many people find counter-intuitive, perhaps even contrary to common sense. Locke’s theory consists of two main logical steps, a definitional distinction between primary and secondary qualities and an application of the principle of economy in explanation. Rejecting the definitions of primary and secondary quality does not remove the absurdity. One could just take Strawson’s response and replace every mention of primary or secondary quality with the appropriate definition in terms of resemblance. The conclusion that we do not perceive objects the way they really are would still follow from the combination of the principle of economy of explanation with the nature of our senseperception.
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This leaves the intuition that we do perceive objects the way they really are in conflict with the principle of economy of explanation. But rejecting the principle of economy
20 It would be possible to challenge the difference between resemblance and non-resemblance. But this would not save the intuition that we have simple ideas directly from sensation that resemble qualities in bodies, which can be rephrased as the intuition that we perceive objects the way they really are. Rather, it would render that intuition meaningless.

7 would open up the possibility of conclusions even more counter-intuitive than the conclusion that we cannot perceive objects the way they really are. For example, an axe-murderer could successfully defend herself in court by arguing that the victim had been decapitated by an ancient curse, and that the movement of the murderer’s axe in between the head and the neck at the moment of decapitation was merely coincidental. If agreement with our intuition is to be the standard of comparison, then we ought to accept the principle of economy and therefore Locke’s theory in order to save many intuitions, even though we have to give up one intuition as a result.
I began by giving a brief interpretation of Locke’s theory of primary and secondary qualities which had the two main results of distinguishing between phenomenal and nonphenomenal primary qualities and of proposing that we should interpret Locke’s statement that secondary qualities are not really in bodies as another way of saying that the ideas produced by secondary qualities do not resemble the configurations of primary qualities which constitute secondary qualities. I then gave an interpretation of Strawson’s response to Locke which used the observation that we can only become aware of bodies and primary qualities through secondary qualities to conclude that we cannot perceive objects the way they really are. This is equivalent to another interpretation I gave, which arrives at the conclusion that we cannot have ideas resembling primary qualities directly through sense-perception. The application of judgement and inference is required. Finally, I suggest that it would be more counter-intuitive to reject
Locke’s theory (and the principle of economy that it contains) than it would be to accept the conclusion that we cannot perceive objects the way they really are.

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Acknowledgements
I would like to thank Nick Gandolfo-Lucia for his advice on how to reduce the first version of this paper to a more modest length and Dylan Verner-Crist for his advice on how to present some of the arguments in this second version.
Bibliography
Locke, John. An Essay Concerning Human Understanding . Edited by Kenneth P. Winkler.
Indianapolis: Hackett Publishing Company, 1996.
Strawson, Peter. “Perception and Its Objects.” In Vision and Mind: Selected Readings in the
Philosophy of Perception , edited by Alva Noe and Evan Thompson, 91-110. Cambridge,
MA: MIT Press, 2002.