Did Berkeley misunderstand Locke’s view of secondary qualities in a way which mattered to his project?
Locke held that there was a distinction between ‘primary qualities’ like extension and solidity, and ‘secondary qualities’ like colour, taste, sound and smell. Berkeley has been charged with misunderstanding both what Locke’s distinction was and how Locke defended that distinction. This essay will defend Berkeley on both counts, and try to put the role of Berkeley’s discussion of the distinction into proper perspective, against those who argue that his alleged confusion about it would be fatal to his immaterialist project.
I begin with the charge that Berkeley misinterpreted what Locke’s distinction amounted to. Both Jackson and Bennett make this accusation, and both agree that Locke thought secondary qualities were powers in mind-independent objects, and that Berkeley missed this. Jackson levels the charge thus: “Berkeley in The Principles of Human Knowledge understands Locke to mean by primary and secondary qualities not imperceptible qualities and powers but perceptible qualities and mind-dependent ideas”. According to Jackson, Locke believes the following: secondary (and tertiary) qualities are powers; powers are not, strictly speaking, qualities; primary qualities are thus the only properties of objects “in the strict sense”; and neither primary nor secondary qualities are perceptible. Attributing this last position to an avowed non-sceptic like Locke may sound implausible, but Jackson means that Locke thought them perceptible only indirectly, by means of perceiving ideas which represented them. These ‘ideas of sensation’ are normally interpreted as what we perceive directly, and as mental rather than material. Yolton (1984) denies that either Locke or Berkeley thought this, but I shall not question the countervailing consensus. Nor shall I question Locke’s view itself; though the last century brought many critics of this once-popular position, understanding the debate between Berkeley and Locke requires me to speak in its terms. I shall outline the evidence perceptual relativity might provide for it, but a full consideration of its advantages and disadvantages is outside the scope of this essay.
Jackson’s central claim is, then, that Locke saw secondary qualities as powers of objects, but that Berkeley thought he saw them as ideas. This is Bennett’s claim too, and what I shall now discuss.
In the next section, I shall argue that Bennett and Jackson provide no good evidence for Berkeley’s having erred. However, their interpretation of Locke is well supported. The Essay concerning Human Understanding calls secondary qualities “nothing in the objects themselves but powers to produce various sensations in us” and “only powers [though] we are apt to take them for positive qualities”; and II.viii.10 does somewhat support Jackson’s contrast between the types of power which constitute ‘secondary qualities’ and real qualities, though I shall not focus on this. Boyle, from whom Locke is widely accepted to have taken the distinction, also explicitly defined secondary qualities as powers to act on the senses.
Bennett shows that this definition is by far the most common in the Essay, citing II.viii.10/14/24, II.xxi.73, II.xxiii.7/9 and II.xxxi.2, and convincingly bringing II.xxiii.10/37 (which might appear problematic) into this fold. A few passages threaten this attribution; here is my unavoidably rapid defusing of these threats:
1) II.viii.15’s claim that only primary qualities resemble our ideas of them is neither obviously a definition, nor incompatible with II.viii.10’s definition (it would be surprising if it were, given their proximity).
2) II.viii.9 claims that primary qualities are found “inseparable from every particle of matter” by “the mind”. McCann plausibly suggests that this is a claim about our concept of matter or body. Bennett agrees, but does not see this as inconsistent with II.viii.10. This is surely right, since II.viii.9 is part of the same set of definitions as II.viii.10. II.viii.13 states that God grants certain material states the power powers to produce ideas of secondary qualities, which suggests that these powers are not inseparable from the concept of body but rather products of God’s free choice. (I ignore the question of whether all primary qualities really are inseparable in this way as irrelevant to the interpretive question.)
3) Locke sometimes says that secondary qualities are not “in” physical objects, implying that they are ideas in the mind. Bennett allows that here Locke has “slipped … to a different thesis”. As Jackson notes, II.viii.8 acknowledges sometimes failing to distinguish between secondary qualities in objects and our ideas of those qualities. Any interpretation should thus respect this distinction. Locke’s slip is natural, for everyday secondary quality words like ‘red’ do double duty, describing both an object’s redness (“that postbox is red”) and phenomenal redness (“this after-image is red”); Locke thinks of these as distinct, non-resembling powers/qualities. II.viii.18 sees Locke compare “sickness caused by manna” to manna’s whiteness, treating both as “nothing but the effects of its operations” – Bennett notes that he should really compare whiteness to emeticness (sick-makingness).
Berkeley’s understanding of Locke’s distinction
Berkeley stands accused of seeing (3) as embodying Locke’s definition of secondary qualities. We have seen that this would be a misinterpretation of Locke, contradicting both his most common definition and his distinction between secondary qualities and our ideas of them (which he himself apologises for sometimes ignoring). What is the evidence that Berkeley made this mistake?
It lies mainly in P9-10 (a discussion of the distinction so brief that Berkeley may not have had time to convey his precise understanding of Locke). This begins: “Some there are who make a distinction betwixt primary and secondary qualities”.
This has been assumed to refer to Locke, but P9-10 do not explicitly mention him, and many others made this distinction, including the Greek atomists, Aristotle, the scholastics, Descartes, Malebranche (whom Berkeley studied early on) and Boyle. We shall later see that Wilson shows that Berkeley, in his discussion of the arguments for the distinction in P14-15, might not have had Locke in mind; if this were so, it would further call into question P9’s target. However, I think it fair to read P9 as targeting Locke, who was much-studied by Berkeley and his audience. Berkeley’s list of primary and secondary qualities echoes Locke’s (though admittedly Locke’s was not unique), and other parts of P9 are also reminiscent of him: “The ideas we have of [secondary qualities] they acknowledge not to be the resemblances of any thing existing without the mind or unperceived; but they will have our ideas of the primary qualities to be patterns or images of things which exist without the mind, in an unthinking substance which they call matter.”
This last passage mirrors Jackson’s interpretation of Locke. That scuppers Jackson’s first attempt to show Berkeley’s error, which ’es the preceding sentence’s description of primary and secondary qualities as “sensible qualities” out of context to suggest that Berkeley thought Locke saw them as directly perceptible. Elsewhere, however, Jackson ’es P10’s genuinely worrisome first sentence: “They who assert that figure, motion, and the rest of the primary or original qualities do exist without the mind, in unthinking substances, do at the same time acknowledge that colours, sounds, heat, cold, and such like secondary qualities, do not, which they tell us are sensations existing in the mind alone, that depend on and are occasioned by [microstructural primary qualities].” Responding to Jackson, Barnes defends this passage as using ‘primary qualities’ and ‘secondary qualities’ as “shorthand expressions for two groups of phenomena” (i.e. phenomenal qualities like afterimages’ redness – an everyday sense of ‘qualities’, which we saw Locke himself use) without treating this as Locke’s normal definition. Berkeley’s language suggests Barnes is right; neither Barnes’ nor Jackson’s interpretation of P10 is demonstrably correct, but we should favour Barnes’ on grounds of charity. Charitable interpretations are not always accurate, but there is particular reason to favour them here: Berkeley was generally a precise and careful (if sometimes overly terse) philosopher; he studied Locke in depth, reading the Essay concerning Human Understanding in his student years; and his notebooks frequently and accurately refer to its doctrines (including those about secondary qualities).
I rest my defence of Berkeley’s understanding of what Locke’s definition was here, but conclude by acknowledging a small deficiency not normally emphasised by his critics: in describing Locke’s views, Berkeley does not distinguish bodies’ powers from the primary qualities on which they rest, as Locke does in moments of precision. However, this distinction is of little consequence to Berkeley’s project, since it does not affect his arguments’ strength or weakness, and (as Barnes points out in Berkeley’s defence) Locke felt able to ignore it in most statements of his position.
Arguments from perceptual relativity
More common than the criticism discussed above is the charge that Berkeley misunderstood the basis of Locke’s distinction between primary and secondary qualities. On this view, Berkeley read Locke as arguing that because our perceptions of secondary qualities vary according to our circumstances, they do not resemble anything in the objects themselves: this non-resemblance is one distinguishing feature of Lockean secondary qualities. As before, I examine first what Locke said, and then what Berkeley thought he said.
Locke offered several arguments for distinguishing primary and secondary qualities, of which Berkeley discusses only some (note that this does not itself reveal misunderstanding). As well as the discussion of perceptual relativity on which Berkeley focuses, various commentators have seen arguments from: the inseparability of primary qualities from the concept of body; the mechanisms which cause perceptions of qualities; the effects of pounding an almond; and the relationship between heat and pain. As is increasingly appreciated, many of these were arguments for Boyle’s corpuscularianism; Locke had worked for Boyle and describes himself as an “under-labourer” to him and other modern scientists. Corpuscularianism involves a primary/secondary quality distinction, so an argument for the former is an argument for the latter. However, it is going too far to claim, as Alexander (1974) does, that corpuscularianism provides Locke’s only basis for the distinction; II.viii.16’s discussion of the relationship between heat and pain, for instance, makes a pre-scientific point.
Regardless, Berkeley does not – as Alexander, Mackie and Mandelbaum imply – miss Locke’s scientific arguments. Hylas describes this “modern way of explaining things” at 2D208, and Philonous replies by denying that it can afford any explanation, for several reasons: corpuscularian science hasn’t fully explained any phenomenon; only spirits can provide causal explanations anyway; and material-mental causation is particularly mysterious (something Locke conceded).
Typically for these commentators, Mackie also claims there is no argument from perceptual relativity. He concedes that there appears to be such an argument in II.viii.21: “the same water, at the same time, may produce the idea of cold by one hand and of heat by the other; whereas it is impossible that the same water, if those ideas were really in it, should at the same time be both hot and cold”. But he notes that II.viii.21 also claims that corpuscularianism can explain this perceptual relativity, interpreting this as an inference to the best explanation. However, it does not follow that this scientific argument is the only one in II.viii.21, and we cannot quite rule out the traditional view that there is also an argument from perceptual relativity. This has appeared to many to be present not only in the passage I ’ed, but also in Locke’s contrast between the relativity of perceptions of heat (a secondary quality) and the alleged constancy of perceptions of shape (a primary quality).
Without endorsing it, Berkeley describes just this sort of argument very accurately in P14: “it is said that heat and cold are affectations only of the mind, and not at all patterns of real beings … [because] the same body which appears cold to one hand, seems warm to another”. As before, he does not mention Locke, and Wilson provides some evidence that Locke may not have been his target: P14 very closely parallels section G of Bayle’s Dictionary’s article on Zeno; Bayle credited Simon Foucher’s Critique de la Recherche de la Vérité with influencing his criticism of the primary/secondary quality distinction; and Foucher published in 1675, before Locke’s 1690 Essay. If (as looks likely) P14 simply repeated Bayle’s criticisms, Locke may not have been the target Berkeley inherited. However, there is a complication: though Popkin sees Bayle’s criticisms prefigured in Foucher, Wilson cannot find them in Foucher’s Critique. Perhaps they occur in Foucher’s other works; however, Wilson’s evidence that Locke may not have been Berkeley’s target is clearly inconclusive, and the considerations I brought to bear when discussing P9-10 count against it.
So let’s suppose that P14 was intended to describe Locke’s argument. We have seen that it is not obviously a misinterpretation of II.viii.21, but, more importantly, it would not damage Berkeley’s immaterialist project even if it was. This is because Berkeley did not argue for immaterialism by extending this flawed argument to cover primary qualities, as he is charged with doing. Instead, he revealed one of its central flaws at P15: “this method of arguing doth not so much prove that there is no extension or colour in an outward object, as that we do not know by sense which is the true extension or colour of the object”. Here, Berkeley is noting that variations in the colours we experience when looking at an object show only that, if we treat that object as material, we cannot ascribe all those colours to it at once without contradicting ourselves. For all P14 shows, the very colour we experience in one set of circumstances (e.g. in sunlight, with healthy eyes) could ‘be’ (i.e. resemble) the object’s “true colour”. A fortiori, P14 does not rule out Locke’s theory that material objects’ secondary qualities are powers which never resemble the ideas of those qualities which they give rise to. Berkeley does not use P14’s argument to rule this possibility out.
In light of P15, Bennett owns himself convinced that his earlier accusation that Berkeley accepted and extended Locke’s argument from relativity was wrong. However, Berkeley’s Three Dialogues provide more fodder for his critics, for the first dialogue argues extensively from relativity, lacks P15’s qualification, and is not ad hominem like P14-15, since Berkeley seems to endorse Philonous’ arguments. This is puzzling, given that the Dialogues were published after the Principles, and so after Berkeley had spotted the limitations of arguing from relativity, and generally take more time in setting out their arguments. Perhaps we can accept Luce’s description of the Dialogues as “not repeating all the detail of the argument”, going along with the received opinion that “the Principles is the definitive statement of Berkeley’s views and the Three Dialogues just a populist reworking”. We could then say that Berkeley simply omitted P15’s qualification from the Dialogues without rejecting it. Leaving open this possibility, I shall consider suggestions that the Dialogues offer an argument different from P14’s.
Stoneham reports, but does not endorse, one such suggestion, based on attributing what he calls the ‘indistinguishability principle’ to Berkeley: “anything indistinguishable from what exists only in the mind can itself exist only in the mind”. Berkeley’s discussion of resemblance in P8 lends some plausibility to this attribution, and a version of the principle could perhaps plug the gap which P15 identifies in arguments from relativity, for reasons best explained in a footnote. Since there is little evidence that the First Dialogue employed the principle for this reason, I shall instead concentrate of Stoneham’s more plausible explanation of how this gap is plugged.
Stoneham thinks Hylas’ acceptance that he cannot provide a criterion to distinguish accurate from inaccurate perceptions suggests that Berkeley thought any such criterion would be arbitrary. This is meant to call into question not only our knowledge of such a criterion, but also its the very existence. On this interpretation, Berkeley is arguing that there is no reason to think that there is one perspective which ‘reveals objects’ true colours’, for this would be arbitrary “favouritism”, as Russell put it.
Philonous does indeed seem to be making this argument when he persuades Hylas to reject his initially plausible candidate for a favoured perspective, “the most near and exact survey”. He notes that microscopes give a closer view than the naked eye, and Hylas finds any suggestion that they reveal true colours unacceptable (apparently because they often depict different colours than the ones he is used to thinking of as real). Stoneham reads Philonous as reminding us that we would need non-arbitrary answers to all these questions: “what certain distance and position of the object, what peculiar texture and formation of the eye, what degree or kind of light is necessary for ascertaining that true colour”. Not only do we lack knowledge of the answers, it seems intuitively odd to claim there even exist answers to all these questions. Of course, not everyone will accept this argument but what matters for present purposes is that Stoneham’s attribution of it to Berkeley is relatively plausible.
The passages I have ’ed suggest that Berkeley was trying to show that nothing like the phenomenal colours and temperatures we immediately perceive could exist in material objects. In showing this, Berkeley thinks he has achieved one of Philonous’ avowed aims: advancing immaterialism by showing that only it can preserve the common sense belief that objects have colours like those we perceive in them. Indeed, Berkeley’s immaterialism comes even closer to common sense on many interpretations, allowing that the very colours we directly perceive (i.e. the quality-instances, rather than just quality-types) belong to objects. Nowadays, such beliefs are often deemed ‘naïve’, but they are part of untutored common sense, and preserving common sense beliefs is, all things being equal, a theoretical virtue.
This is not the only way Berkeley’s arguments from relativity can genuinely aid immaterialism. They also contribute to a major part of his project in the Dialogues and (especially) the Principles: robbing us of ways to understand mind-independent existence, so that we end up with no positive conception of it. Many elements of the Principles can be seen to fit into this project. The introduction’s lengthy attack on the conceivability of abstract qualities (like extension-in-general, which some definitions of matter had involved) makes sense when we read Berkeley as arguing that matter is inconceivable. P1-7 discuss the concept of a physical object’s existence, and argue that it involves being perceived. P8 rejects the suggestion that ideas can resemble (and so represent) anything mind-independent. P9-13 argue that certain concepts which might be thought to describe matter either cannot do so, or are unacceptably abstract. P16 and other passages consider characterisations of matter in terms of its relations (causal or otherwise) to other things. P80 rejects a “negative definition of matter”, which Berkeley anticipates will be the materialist’s last resort. Many of these arguments are also found at 2D215-223. Together, they suggest that we have no conception of matter.
We can now see how Berkeley’s rejection of the primary/secondary quality distinction fits into his project: he is trying to show that materialists cannot preserve the common sense belief that tables can have colours and shapes resembling those we directly perceive, so that these phenomenal qualities cannot be used to characterise matter. Using our understanding of Berkeleyan arguments from the relativity of our perceptions of secondary qualities, let us now see how Berkeley applies “the same arguments” to primary qualities.
Primary qualities and perceptual relativity
Locke offers a “perhaps” exhaustive list of bodies’ primary qualities at II.xxi.73: (1) extension, (2) solidity, (3) mobility and duration, (4) number and (5) existence. We have already seen that Berkeley’s whole project (of which we are considering only a part) involves showing that nothing can exist mind-independently. But he also emphasises the relativity of our perceptions of the other qualities on this list, as follows.
(1) P14 points out that “to the same eye at different stations, or eyes of a different texture at the same station, [figure and extension] appear various”. In the first Dialogue, Philonous offers similar examples, like a mite’s perspective. At 1D189, he considers shape as well as size, mentioning that just as water can seem hot to one hand and cold to another, an object can seem small and round to a naked eye and large and angular to one looking through a microscope.
(2) At 1D191, Philonous equates our experience of solidity with our experience of resistance, which is relative to our strength.
(3) P14 claims that “if the succession of ideas in the mind become swifter, the motion, it is acknowledged, shall appear slower”. This means that duration and perceived speed of motion are likewise relative.
(4) P12 makes the Fregean point that number is ‘relative’ to what we are counting. (This does not allow a typical argument from relativity, and fails to show that, once we have decided to count apples rather than atoms, we must deny there is a mind-independent number of apples.)
I shall focus on Berkeley’s discussion of extension. His critics claim this involves the misapplication of a misunderstood argument from Locke. But we can now see this to be false. I have argued that even if Berkeley mistakenly thought Locke had offered the argument described in P14, he rejected this argument himself, and so did not ‘misapply’ it. I have also argued that if the Dialogues endorse a stronger argument from perceptual relativity, it is most probably the one described by Stoneham. There is no evidence that Berkeley thought Locke had offered this argument. Its task is to take us from Locke’s concession that our ideas of extension are distinct from material extension itself to Berkeley’s denial of Locke’s claim that the former could resemble the latter. To do so, it need only show that these ideas depend on perspective in a way which makes it implausible that some of them (e.g. those perceived by a mite) resemble an object’s true material extension and others do not. This would refute Lockean materialism, which Berkeley correctly understands to require that each object has a determinate material extension which cannot resemble all the different ‘ideas of sensation’ yielded by different perspectives on it. (It would not refute modern materialisms which do not require this, either rejecting Locke’s notion of ‘resemblance’ altogether or anachronistically treating it as a weak structuralist notion.)
Since this argument involves no misunderstanding of Locke, it is not my task to consider whether it works. But even a Lockean materialist might have several comebacks, and I shall consider one example: the suggestion that though there is no common measure of colour, rulers provide a common measure of extension. Mites and men could both see that a given object is the same length as a given ruler. I mention this comeback because it is fairly plain what Berkeley’s response would be: he would say that while the object may always appear the same length as a metre-long ruler held up against it, the size of both in our visual field will vary as we move closer or farther. This size is what constitutes phenomenal extension, the quality which Locke and Berkeley think we directly perceive. We might, of course, want to abstract the rod’s shape size relative to the ruler held against it (and its shape) from its absolute size in our visual field. But then our quarrel with Berkeley lies in his discussion of abstraction, not perceptual relativity. Something similar is true of many objections to Berkeley’s discussions of perceptual relativity and primary qualities; the real problems often turn out to concern quite separate arguments and presuppositions which he makes.
We have seen that Berkeley understood both the nature and the scientific basis of Locke’s distinction between primary and secondary qualities. The claim that the Principles falsely ascribe an argument from perceptual relativity to Locke is questionable, but even if it were true it would not matter to Berkeley’s project, because he rejects that argument. If he accepts a separate argument in the Dialogues, it is not one that he ascribes to Locke. Berkeley is not always thinking of Locke, and Hylas is not simply Locke’s representative; later philosophers have misinterpreted both Locke and Berkeley by forgetting this, and seeing one through the lens of the other (which makes for neat histories of philosophy, but inaccurate ones). Locke and Berkeley have different aims, and different arguments. Properly understood, Berkeley’s discussion of primary and secondary qualities involves no obvious misunderstanding, and serves the purposes of his own particular project.
Listed below are the works I have referred to directly in this essay. I have given the original dates of publication, and, where this is different, the date of publication for the volume I have used for page references: in the text, I have given the original date of publication, to avoid anachronisms like ‘Boyle 1999’. In standard fashion, I have referred to section 1 of Chapter I of Book I of Locke’s An Essay concerning Human Understanding as I.i.1; to section x of Part I of the Principles of Human Knowledge as Px; to page y of Dialogue x of the Three Dialogues Between Hylas and Philonous (in Volume II of the Luce-Jessop edition of Berkeley’s Works) as xDy; and to entry x of Berkeley’s Philosophical Commentaries as PCx.
Aaron, R.I. (1955). John Locke (OUP)
Alexander, Peter (1974). ‘Boyle and Locke on Primary and Secondary Qualities’, reprinted in Tipton (ed.), Locke on Human Understanding (OUP, 1977)
Barnes, Winston H.F. (1940). ‘Did Berkeley Misunderstand Locke?’, in Mind 49, 52-57.
Bennett, Jonathan (1965). ‘Substance, Reality and Primary Qualities’, reprinted in Martin and Armstrong (eds.), Locke and Berkeley (Doubleday, 1968)
Bennett, Jonathan (1971). Locke, Berkeley, Hume (OUP)
Bennett, Jonathan (2001). Learning From Six Philosophers, Vol. 2 (OUP)
Boyle, Robert (1666). ‘The Origin of Forms and Qualities’, in Hunter and Davis (eds.), The Works of Robert Boyle, Vol. 5 (Pickering & Chatto, 1999)
Cummins, Philip D (1966). ‘Berkeley’s Likeness Principle’, reprinted in Martin and Armstrong (eds.), Locke and Berkeley (Doubleday, 1968)
Curley, E.M. (1972). ‘Locke, Boyle, and the Distinction between Primary and Secondary Qualities’, in Philosophical Review 81, 438-464
Jackson, Reginald (1929). ‘Locke’s Distinction between Primary and Secondary Qualities’, in Mind 38, p56-76
Luce, A.A. (1949). Life of George Berkeley (Thomas Nelson)
Luce, A.A. & Jessop, T.E. (eds.) (1948-1957). The Works of George Berkeley, Bishop of Cloyne (Thomas Nelson)
Mackie, J.L. (1976). Problems from Locke (OUP)
Mandelbaum, M. (1964). ‘Locke’s Realism’, in his Philosophy, Science, and Sense Perception (Johns Hopkins Press)
McCann, Edwin (1994). ‘Locke’s Philosophy of Body’, in Chappell (ed.), The Cambridge Companion to Locke (Cambridge University Press)
Popkin, Richard H. (1967). ‘Skepticism’, in Paul Edwards (ed.), The Encyclopaedia of Philosophy, Vol. 7 (Macmillan and The Free Press)
Robinson, Howard (1994). Perception (Routledge)
Robinson, Howard (ed.) (1999). Principles of Human Knowledge and Three Dialogues (OUP)
Russell, Bertrand (1912). The Problems of Philosophy (OUP paperback, 2001)
Stoneham, Tom (2002). Berkeley’s World: An Examination of the Three Dialogues (OUP)
Stroud, Barry (1980). ‘Berkeley v. Locke on Primary Qualities’ in Philosophy 55, 149-166
Wilson, Margaret D. (1982). ‘Did Berkeley Completely Misunderstand the Basis of the Primary-Secondary Quality Distinction in Locke?’, in Turbayne (ed.), Berkeley: Critical and Interpretive Essays (Manchester University Press)
Yolton, J.W. (1984). Perceptual Acquaintance from Descartes to Reid (Blackwell)
 Best defined as objects which can exist without minds existing – those who think God’s existence is necessary (and perhaps also that every object’s continued existence depends on God) will count as denying mind-independent objects by this definition, but there is no easy way around this.
 Typically defined as powers of objects to act on other objects, rather than on our senses; II.viii.10 calls these ‘secondary qualities’ too, but as they are not at issue here I shall reserve the term for colours, sounds, etc.
 Jackson 1929:62 claims this is a sense of ‘primary’ – this is a stretch, but he does not rest his case on it.
 Jackson’s calling indirectly perceptible things ‘imperceptible’ echoes Hylas’s claim at 1D174 that “in truth the senses perceive nothing which they do not perceive immediately”. At this early stage of the Dialogues, Hylas is often thought to concede only what Berkeley thought a Lockean would. However, the important point is not whether we call indirect perception ‘perception’, but that most scholars agree with Jackson that Locke denied direct perception of even primary qualities.
 Cf. Robinson 1994:19. Robinson himself interprets them as mental images or “sense-data”; however, I do not myself mean to claim that Locke accepted all aspects of modern sense-datum theories – or of the ‘veil of perception’ view attributed to him in Bennett 1971, for that matter.
 Which still has defenders: see, for example, Robinson 1994.
 Bennett 1971:112.
 Bennett 2001:76-77, has the details, too lengthy to describe here, but relatively unproblematic.
 It is often unclear which claims about primary and secondary qualities Locke intends to be definitional truths as opposed to rough-and-ready tests for them, contingent facts about them, etc. Admittedly, the same goes for the passages I have just cited, though II.viii.10 is phrased like a definition, so should be allowed primacy over claims which are not.
 II.viii.17/18/23, II.xxxi.2.
 2001:77. Bennett’s italics.
 Some modern philosophers dislike talk of ‘visual images’ and ‘phenomenal qualities’, but as I said I shall not discuss their criticisms. For brevity, I use the term ‘phenomenal qualities’ to refer to Berkeley’s ‘sensible qualities’, which are aspects of Lockean ‘ideas of sensation’. (When talking of these ‘qualities’, it should be understood that both Locke and Berkeley were nominalists.)
 E.g. by Robinson 1999:213.
 Jackson 1929:72; for a rebuttal, see Barnes 1940:53.
 Stroud 1980, footnote 2, lists philosophers who make this charge; footnote 1 lists philosophers who have genuinely made the same error Berkeley is accused of.
 These are, for instance, described in Mackie 1976:17-24.
 cf. Alexander 1974, Curley 1972, and Mandelbaum 1964.
 Aaron 1955:12-14.
 ‘Epistle to the Reader’, Essay Concerning Human Understanding.
 Made previously by Aristotle, who was no corpuscularian.
 Cf. 3D257.
 See Stroud 1980:158.
 E.g. Robinson 1994:35.
 These are unfortunately not available in Oxford’s libraries.
 Stroud (1980) offers a close reading of the early Principles to show how little weight is put on P14. He also argues that Berkeley cannot have thought that P14’s epistemological argument was Locke’s basis for the metaphysical primary/secondary quality distinction. (Stroud 1980:157.) We shall see that the Dialogues arguably offer a separate, metaphysical argument.
 As described in Stoneham 2002:viii. Stoneham counters that the Dialogues could equally be taken as the more mature work.
 Stoneham 2002:67.
 Put briefly: suppose that the colour I experience when looking at an object differs from the colours it presents in different circumstances in ways that convince me that that colour exists only in my mind, and that no resembling colour exists in the object. Then whenever I experience an indistinguishable colour, the indistinguishability principle would on one interpretation have me conclude that nothing resembling it exists in the object. Since this is likely to happen at least once for every shade I perceive, I would conclude that all colours can only exist in the mind.
 Where a Lockean or Berkeleyan quality-type is something like a modern ‘resemblance class’ of quality-instances. Stroud is sometimes unclear about this distinction, as when he says :“Berkeley thinks that facts about the ‘relativity’ of perception can be used to prove that perceived colour or perceived extension do not exist without the mind’…” (1980:162). If Stroud is merely talking about directly perceived quality-instances, he understates the ambition of the Dialogues’ arguments from perceptual relativity, at least by Stoneham’s interpretation.
 This is not his only list.
 1D184-185. At 1D189, Philonous considers shape as well as size, mentioning that just as water can seem hot to one hand and cold to another, an object can seem small and round to one eye and large and angular to another which is looking through a microscope.
 Phenomenal extension is arguably susceptible to structuralist treatment in a way that phenomenal colour is not.
 Albeit in proportionate fashion, with the ruler ‘enlarging’ at the same rate as the rod. This has led to the post-Lockean suggestion, to which I alluded earlier, that phenomenal extension in our visual field has a similar structural role to material extension in the external world.