What is it about Lemons?


Barry Stroud is one of the most important and respected philosophers of his generation. His work is a “long-standing and ongoing quest to understand philosophy… One assumption that Stroud endeavors to unsettle from the mind and place on the page is the very idea that we know what we are after when we do philosophy.” Jason Bridges and Niko Kolodny, The Possibility of Philosophical Understanding: Reflections on the Thought of Barry Stroud  OUP 2011.

A great introduction to his thought is the review by Tom Nagel of one of Stroud’s best books, The Quest for Reality: Subjectivism and the Metaphysics of Colour  OUP 2000. It’s amazing that something as simple as color attracts so much attention in philosophy. Simply put, the problem is this:  physics seems to say that color does not exist in the physical world. No ordinary object is colored. Light has no color. Yet we see color all around us. Do the colors we see occupy another realm, an immaterial mental realm? That would require splitting the mind and body, and commit us to a nasty form of dualism. Few contemporary philosophers want to accept those consequences. But how can we get color back into the physical world without getting into a conflict with physics?  

Colors are of philosophical interest for a number of reasons. One of the most important reasons is that color raises serious metaphysical issues, concerning the nature both of physical reality and of the mind. Among these issues are questions concerning whether color is part of a mind-independent reality, and what account we can give of experiences of color. These issues have been, and continue to be, inextricably linked with important epistemological and semantic issues. https://plato.stanford.edu/entries/color/ 


The essence of Stroud’s argument, as he put it later in response to criticism, is that


one must believe that some objects are colored in order to attribute color perceptions and beliefs about the colors of things to others. The necessity that attributed beliefs be shared does not alone imply that any of those beliefs are true… This puts at the center of my argument the conditions of the attribution of psychological states whose contents involve the colors of things… I argue that any attributor must hold some categorical beliefs about the colors of objects in order even to recognize the contents of those perceptions and beliefs involving colour that he acknowledges to be part of the world. And that is inconsistent with believing that no objects at all are colored.  Philosophy and Phenomenological Research, Vol. 68, No. 2 (Mar., 2004), pp. 431-432

Tom Nagel does a fine job of explaining Stroud’s argument and particularly in explaining the background and significance of it. BQTA Contributor Rick Lenon, M.D. and I will have a lot to say about all of it later. For now, here is Nagel, uninterrupted.


Quest cover 9780195133882


What is it about lemons?

London Review of Books   20 September 2001


Thomas Nagel

  • The Quest for Reality: Subjectivism and the Metaphysics of Colour by Barry Stroud
    Oxford, 228 pp, £19.99, January 2000, ISBN 0 19 513388 9

This strange and absorbing book sets out to undermine the central metaphysical ambition which has dominated philosophy since the 17th century – that of reaching what Bernard Williams calls an ‘absolute conception of reality’. The aim is to achieve a comprehensive understanding of the world, consistent with modern science, which distinguishes between what exists objectively, independent of our minds, and what is merely subjective – due to the effects of the world on our minds and our responses to it.

In resisting the metaphysical conclusions that result from this quest, Barry Stroud writes against the temper of the times, and very much in the spirit of the later Wittgenstein, who was also self-consciously out of step with the times, and who remains for the most part unassimilated by contemporary philosophy, in spite of being conventionally venerated as one of the few great philosophers of the 20th century. Stroud’s philosophical style, however, except for its lack of ornament and strict avoidance of technical language, is completely unlike Wittgenstein’s, which was gnomic and indirect. Stroud is clear, explicit, methodical and relentless. He tries to block every exit, and to say exactly what has been shown and what has not. The result is deliberately frustrating, for his aim is to baffle a desire for understanding of our true relation to the universe that is at the root of philosophy and that Stroud himself recognises we cannot get rid of.

Like all of his writings – on scepticism, on Hume, on Kant, on Wittgenstein – The Quest for Reality displays a profound grasp of the history and logical structure of philosophical problems and theories, and a feeling for the derangement of thought that underlies them. He insists that the understanding sought by metaphysics is distinct from scientific understanding, for we could attain it only by answering a question which remains after all scientific results are in. Yet the metaphysical outlook he wants to resist arose and continues to be widely accepted because it seemed to so many people obvious from the beginning that the results of modern physical science reveal a distinction between subjective appearance and objective reality, one which demands formulation as a comprehensive philosophical worldview. Before explaining why Stroud believes that this is not the case, let me describe the view more fully.

The new science of the 17th century was brought into existence when Galileo and Newton developed a quantitative geometrical understanding of the physical world and the laws governing it, a description that left out the familiar qualitative aspects of things as they appear to the separate human senses: their smell, taste, sound, feel and colour. Colours and smells did not enter into physics, and in spite of the look and aroma of a typical chemistry lab, they didn’t enter into chemistry either, when it subsequently developed into a theory of the true composition of everything around us from a limited number of elements. Stroud quotes Galileo: ‘If the ears, the tongue, and the nostrils were taken away, the figure, the numbers and motions of bodies would indeed remain, but not the odours or the tastes or the sounds, which, without the living animal, I do not believe are anything else than names.’ This view was taken up by Descartes, and then enshrined by Locke as the now familiar distinction between primary and secondary qualities – the primary qualities of size, shape and motion being those that belong to things as they are in themselves, and the secondary qualities of colour, sound, taste, feel and smell being mere appearances, produced by the action of these things on our senses.

This conception of the world, as Stroud says, ‘came to seem like nothing more than scientifically enlightened common sense’. And it has survived changes in physical science that have long since rendered obsolete the original catalogue of primary qualities. A modern Locke has to accommodate charge, spin, superstrings and space-time of many more than three dimensions, but the idea is the same: the physical world as it is in itself is describable in quantitative, spatiotemporal terms; everything else we say about it depends on how it affects us or how we react to it. Objective, mind-independent reality is the now totally unfamiliar world described by a rapidly developing physics; the familiar world that we live in, from colours to values, is subjective and mind-dependent.

Stroud attacks this ‘unmasking’ form of explanation, as he aptly names it, through what has always been its most seductive example, that of colour. It is very easy to agree to the philosophical commonplace that ripe lemons are not really yellow in themselves: they just look yellow to human beings, and the explanation for this is that their surface reflects light in such a way that when it strikes a human eye, it produces the characteristic visual experience that we call seeing yellow. The reflective properties of lemons are due to their primary qualities, describable entirely in the language of physics – chemical surface structure, photons, quantum theory – without any reference to colour.

Physics does not purport to explain the operations of the human mind, but those who accept the standard view that colours are subjective base it on a rough idea that is not actually an explanation, but rather a belief that the true explanation has a certain form. This rough idea is that physics accounts for everything that happens when light is reflected from a lemon to a human eye, thus triggering physical effects on the retina which in turn produce physical effects in the brain – and the brain, in some way we don’t yet understand, produces a visual experience. Nothing about colour enters into the first part of the story, which is pure physics.

By contrast, when we see a lemon, its actual physical shape is part of the explanation of the pattern of reflected light it throws on the retina, and therefore of our visual perception of it as ovoid. So lemons really have the shape they appear to have, but not the colour. Or perhaps, alternatively, we could say that colour can be ascribed to lemons but only in a dispositional sense: their yellowness consists in the fact that they are disposed to affect human vision in a certain way. In any event, the conclusion that lemons are not really in themselves yellow seems very like a direct consequence of modern science.

Stroud has a great deal to say about this picture. He points out, first of all, that nothing about the colours of things follows from physics because physics simply doesn’t mention colours. It describes the specifically physical properties of things and the relations among them, but does not say that these are all the properties there are. To reach the conclusion that physical objects are not really coloured requires a further, philosophical step. We have to separate out from the world described by physics our perceptions of and beliefs about the colours of things, and determine that the most plausible account of these phenomena does not involve the attribution of colour to physical objects.

Barry Stroud

Stroud seems right to reject dispositional analyses of colour. Nothing could have the disposition to affect human observers in the typical way if there were no such thing as vision. Dispositional analyses imply that if creatures with vision had never been on the evolutionary menu, nothing would ever have had any colour at all. This seems inconsistent with the meaning of the word: there may be an independent metaphysical argument to show that gold is not yellow, but if it’s yellow now, it was yellow before there were any human beings and would have been yellow even if creatures with vision or living organisms were not among the possibilities of nature. The dispositional analysis of colour fails, because it is really equivalent to the elimination of colour from the external world.

But Stroud’s main claim against the unmasking project is much more fundamental, and philosophically radical. He believes that the theory is an answer to a question that we cannot ask. The reason is this. To find out whether it is possible to carve out from the totality of the things we ordinarily believe a subset – the description in terms of primary qualities – which tells what the external world is really and objectively like, we have to begin by acknowledging the appearances to the contrary. The whole point of the unmasking explanation is to show that those appearances are merely subjective. So at the start of the investigation, we have to recognise that people have visual perceptions of colour and beliefs about the colours of things, while suspending judgment about the truth of those perceptions and beliefs, in order to see whether we can develop an account of the situation which dispenses with their objective truth. But this, Stroud says, is something we cannot do.

His arguments depend on a view of psychological concepts that he shares with Wittgenstein and Donald Davidson:

We identify what different people think, believe and perceive in ways that are as rich and complex as our conception of the nonpsychological world onto which those thoughts, beliefs, and perceptions are directed . . . we who inhabit the world can understand someone in that world as believing something or as perceiving something only if we can somehow connect the possession of the psychological states we attribute to the person with facts and events in the surrounding world that we take the beliefs and perceptions to be about.

These arguments, he says,

strongly suggest that no one could abandon all beliefs about the colours of things and still understand the colour terms essentially involved in ascribing perceptions and beliefs about the colours of things. If that is so, no one competent to understand and acknowledge the perceptions and beliefs he hopes to unmask could free himself completely from all commitment to a world of coloured things. So no one could succeed in unmasking all those perceptions and beliefs as giving us only ‘appearance’, not ‘reality’.

Although individual colour beliefs and impressions can be false, Stroud holds that we cannot ascribe them at all unless we believe that many of them are true, because we couldn’t give any content to the belief or impression – we couldn’t identify it – without some conception of its object. The unmasker can’t separate out these psychological states as pure subjective events, detached from the world. He can’t understand them apart from the world that they are about.

However – and this is the strangest part of his view – Stroud does not think that the existence of colour beliefs implies that any of them are actually true; his thesis is only that we cannot believe that there are colour beliefs unless we believe that many of them are true.

He likens the situation formally to what is known as Moore’s paradox (after G.E. Moore, who identified it): You can’t consistently assert, ‘I believe it is raining but it isn’t,’ even though there is no inconsistency in your believing that it is raining when it isn’t. According to Stroud, the unmasker likewise cannot consistently assert: ‘People believe objects are coloured but they aren’t.’ But this doesn’t mean it would be contradictory or inconsistent for that to be so. It means only that he cannot pose the question which the unmasking explanation tries to answer, by admitting colour appearances and then asking whether there are colours: In specifying that people have colour beliefs and perceptions he is already committed to a world with colour in it.

Furthermore, because the question can’t be asked, we can’t reach a positive answer either – namely that colour is objectively real. Colour can neither be unmasked as merely subjective, nor metaphysically endorsed as objectively real: beliefs about colour must remain at their non-metaphysical, ordinary level. We know that lemons are yellow because we can see that they are, and that’s the end of it.

But are they really yellow? Eat your broccoli and don’t ask so many questions. This is, as I say, a frustrating argument. It tells us we can’t ask a question we strongly wish to ask in order to understand our relation to the world in a fundamental way – and that we are deluding ourselves when we think we have posed it, let alone answered it.

Demonstrations of the limits of thought are an important element in philosophy, and Wittgenstein tried to turn philosophy into a method of showing that most of it consists of doomed attempts to violate those limits. Stroud is much more cautious. He insists that each case must be considered on its own, and does not claim that the detailed argument he offers for the case of colour can be transferred even to other secondary qualities, like sound, let alone to more distant examples, like value. But he clearly suspects that unmasking will not work in the important cases.

He briefly discusses the case of value – another prime candidate – and suggests that it may be impossible to attribute any value judgments to people without committing oneself to the truth of some of them. The problem again is what is needed to understand their content as judgments of value, rather than, say, mere desires or aversions. And if we cannot even admit the existence of value judgments while suspending judgment as to whether any of them are true, we cannot get into the position to ask whether values have objective reality or are merely subjective appearances.




If Stroud is right, we are left with a world consisting of all the different kinds of thing we take to be true – full of yellow lemons, stock-market rallies, beautiful landscapes and wicked dictators, as well as quarks, neurotransmitters and black holes. These things are very different from one another, and we come to know about them in different ways, but there is no ground for singling out some of them for the title of objective reality.

This is an absolutely fundamental issue: can large segments of our everyday view of the world and what is true of it be separated out and given a purely psychological interpretation? It arises everywhere in philosophy. Unmasking interpretations have been offered even for the primary qualities and matter (by Berkeley, who thought that the shape we see is as subjective as the colour), for mathematics, aesthetics, causality and of course ethics. Stroud is pressing a question that threatens to stop metaphysics before it starts, leaving us with most of our scientific and prephilosophical beliefs, and familiar methods of justifying and correcting them, but no overarching theory of reality.

I cannot believe that he is right, though much of what he says is convincing. One thing he seems clearly right about is that when you look at a lemon and it looks yellow to you, there is no way of correctly describing your mental state without talking about colour as a property which, if it exists, is a property of physical objects. The lemon looks to you to be yellow. This cannot be redescribed as the lemon’s producing in you a yellow visual sensation. As Stroud says, it is not like perceiving that a thumbscrew is a painful instrument by feeling pain when it is applied to your thumb. Thumbscrews would not be painful if no one could feel pain: their painfulness is nothing but a disposition to cause pain under certain conditions. But the yellow in your visual experience is just the colour that the lemon appears to have. It looks to be yellow fully as much as it looks to be ovoid.

The question is whether we can acknowledge this while leaving open the possibility that all visual appearances of yellow or any other colour are a kind of illusion – the illusion of perceiving a property that nothing has, a property that does not exist. It would have to be a natural illusion shared by all humans with normal sight, like other optical illusions, and it would have obvious utility, enabling us to identify and classify objects by sight, because the illusion of different colours is naturally produced in us in a systematic way by reflected light.


We can certainly see colours where nothing coloured exists – Stroud gives the example of the rainbow. And in familiar optical illusions people see differences of length or size where there are none. But as ordinarily understood, the intentional objects of these perceptions (i.e. what they appear to be of) are properties which, though not present in the particular case, have plenty of other true instances. No coloured object is seen when I see a rainbow, but the question is whether this could be true of all colour vision. Could the object – the intentional object – of a perception or belief be that something has a property which does not and never did exist, which could not be possessed by any object, and which is also distinct from a whole range of other properties of the same type, none of which exists?

It seems to me that the best reason for thinking  this is still the story that physics tells about vision, surfaces and light. One of the things that must be true if lemons are yellow is that their yellowness explains why they look yellow. This means that their being the colour they look to us to be must explain the psychological effect. But this seems to be incompatible with what we know about the physical facts. We are pretty sure that what lemons contribute to the explanation of vision are just those physical properties that cause them to reflect light in certain ways; the rest of the explanation, which we don’t have, is about the relation between the brain and the mind, and doesn’t include anything about lemons. So unless colour is really a physical property of surfaces, describable in terms of their primary qualities, it isn’t part of the explanation of what we see.

But yellowness – the property lemons appear to have – is, if it is anything, a property in addition to all their primary qualities. Since no such property of lemons plays a role in the explanation of their looking yellow to us, it is an illusion – one with which we’re all familiar and which we can identify in ourselves and others through the systematic similarity of the circumstances under which it occurs. Stroud believes that admitting the possibility of a colourless world would require us also to give up the assumption that people see things as coloured; I remain unpersuaded.

As Stroud says, no argument in this difficult territory can hope to be final. And as he also says, the question has to be investigated separately for each case: note, for example, that there is no ‘rival explanation’ argument from physics that could support an unmasking strategy about ethics – unmasking there is more likely to take the form of the reinterpretation of ostensibly objective value judgments as personal feelings. I don’t think this will succeed, but again I am sceptical that the question can be stopped before it starts. Whatever one thinks of the conclusion, it is illuminating to think through the argument. This is philosophy of an exemplary purity, tenacity and depth.


Tom Nagel


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