In chapter 6 of his ‘The Mind’s Eye’ Oliver Sack wrote up his diary entries of his experiences after he had been diagnosed with an ocular melanoma. Sacks had radiation treatment to try to kill the melanoma. After the treatment he had his damaged eye covered with a thick patch. He reported the following strange experience he noticed few days after his treatment:

“If I have been looking at something and then close my eyes, I continue to see it so clearly that I wonder whether I have actually closed my eyes. A startling example of this happened a few minutes ago when I was in the bathroom. I had washed my hands, was staring at the washbasin, and then, for some reason, closed my left eye. I still saw the wash basin, large as life. I went back into my room, thinking that the dressing over the right eye must be absolutely transparent! This was my first thought and, as I realised a moment later, an absurd one. The dressing was anything but transparent- it was a great wodge of plastic, metal, and gauze half an inch thick. And my eye, beneath it, still had one of its muscles detached and was in no position to see anything. For the fifteen seconds or so that I kept my good eye closed, I could not see anything at all. Yet I did see the washbasin- clear, bright, and as real as could be.” (The Mind’s Eye, p. 160)

In the Analytic Philosophy Facebook page Tom Clark (Creator of the Facebook Naturalism page) used a hallucination of music produced by the same brain area as perception of music to illustrate what he thinks the hard problem of consciousness is. Tom’s conclusion on the topic is that the music hallucination shows that our experiences are internal in both: experiences of real world objects and of hallucinations of real world objects. He argues that this makes it very clear what the hard problem of consciousness is: how does the grey matter in our brains produce our whole worlds of experience?  So while we can explain what the neural correlates of an experience are we still haven’t explained how that experience arises. I think that Sack’s experience also makes clear the supposed internal nature of experience and hence what the hard problem of consciousness is.

Dennett famously discussed the nature of hallucinations in his ‘Consciousness Explained’ pp. 3-18. I think that it is worth considering what Dennett’s views on hallucinations are and how he would try to handle Tom’s evidence. Tom is of course a former PhD student of Dennett’s so he knows his work well, and he obviously isn’t impressed with Dennett’s arguments on the topic. So this brief blog is for my own clarification rather than a criticism of Tom. However I would welcome any feedback from Tom on the topic.

Dennett of course does not deny that we can experience hallucinations of music on p. 4 of ‘Consciousness Explained’ he makes the following point:

“Not wanting to horrify you, then, the scientists arrange to wake you up by piping stereo music (suitably encoded as nerve impulses) into your auditory nerves. They also arrange for the signals that would normally come from your vestibular system or inner ear to indicate that you are lying on your back, but otherwise, numb, blind. This much should be within the limits of technical virtuosity in the near future-perhaps possible even today.” (Consciousness Explained, p. 4)

Dennett is discussing the possibility of scientists being able to create virtual worlds that could fool us. He does not doubt that we could be fooled by musical hallucinations. So he would presumably have no problem with the article that Tom Clark shared, obviously though he wouldn’t draw the same conclusions though.

Dennett argues that it is pretty much computationally impossible to build a brain in a vat that could be fooled into thinking that it is a subject walking around the world experiencing this and that. He admits that there are some cases where we experience mild hallucinations like phantom limbs, after images etc. However he thinks that strong hallucinations of the brain in a vat kind are pretty much impossible. He defines a strong hallucination as “a ghost that talks back, that permitted you to touch it, that resisted with a sense of solidity, that cast a shadow, that was visible from any angle so that you might walk around it and see what its back looked like” (ibid p. 7)

Now in the Sacks quote above he describes seeing a scene that is indistinguishable from reality. However notably he takes a passive attitude towards the scene he doesn’t move to see if it changes perspective, he doesn’t try to touch the scene etc. Sacks attitude is one that Dennett claimed was typical of people experiencing hallucinations:

“But that review also provides a clue leading to another theory of the mechanism of hallucination-production: one of the endemic features of hallucination reports is that the victim will comment on his or her unusual passivity in the face of the hallucination. Hallucinators usually just stand and marvel. Typically, they feel no desire to probe, challenge or query, and take no steps to interact with the apparitions.” (ibid, p. 9)

It is interesting that when Sacks had his hallucination, which he claimed lasted for 15 seconds he stood passively looking at the hallucination without moving about and probing it just as Dennett’s model predicts. Dennett argues that hallucinations rely on mechanisms of the brain “expecting” us to only interpret the data along certain lines. So Dennett would presumably think that the article posted by Tom Clark does not really effect his position at all. However Tom Clark would presumably reply that on contrary his example refutes Dennett’s position. The hallucinations while not strong in Dennett’s sense still are reported to occur by subjects and brain imaging supports some of these reports. Since on pain of breaking Leibniz’s law these experiences cannot be identical with particular brain states we do indeed seem to be faced with a hard problem.

The problem is not unique to musical hallucinations. It is well known that two thirds of occipital lobe are lit up when people report experiencing mental imagery and Dennett has written about this (see his 2003 Reply to Pylyshyn). However he does not draw the same conclusion as Clark. The reason for this is that according to Dennett it only SEEMS to us that we are experiencing these hallucinations and mental Images; we are not actually experiencing them. Clark on the other hand argues that we manifestly are experiencing these hallucinations, it is not that merely that it seems to us we are. This is the crux of the debate between Clark and Dennett. Not whether people hallucinate, but whether hallucinations are “Real Seeming” painted in Cartesian theatre by some kind of ghostly substance. I will briefly discuss Dennett’s take on the Colour Phi illusion to illustrate his notion of real seeming.


In his Content and Consciousness Dennett talks about the fact that while it may seem to some people that they have mental imagery, closer examination reveals that what they call a mental image is really only a description. Twenty five years later in his Consciousness Explained when discussing Kosslyn’s experiments on mental imagery, Dennett noted that despite appearances mental imagery is really all tell and no show. One curious thing about Dennett’s view is the fact that he claims that despite the way things seem mental imagery is really a mental description. What is strange about this view is the fact that a description can seem like an image. This is a very odd way to understand the word ‘seem’. A paradigm example of an x seeming like it is a y is given by Descartes. He talks about how a stick which is in transparent water will seem to be bent because of light refraction, though in reality the stick is not bent. What Descartes means by the words ‘seems to be’is the same as ‘appears to be’; and this of course is the standard meaning of ‘seems to be’. However, even to a weak imager like me, it is patently obvious that mental images are nothing like mental descriptions. If something really seemed (as in appeared) to me like an image, then it follows that I would have an experience of something image-like, and a description is in no way image-like. This leads to the question of what Dennett could possibly mean when he admits that it at least seems to some people that they experience mental images?

In Consciousness Explained Dennett carefully explains what he means by the word ‘seems’; evaluating his views on this will help clarify his strange beliefs about the nature of images. In Chapter 5 Section 5 of Consciousness Explained Dennett discusses the colour phi experiment[2]. In this discussion he makes explicit his strange views on the nature of ‘seeming’.  The colour phi phenomenon is apparent motion. We see examples of it on our television screen every day, where a series of still pictures are flashed one after the other at a certain speed to create the illusion of motion. Dennett discusses a simple example of colour phi where two spots separated by as much as 4 degrees of visual angle are flashed rapidly, creating the illusion of one spot moving back and forth rapidly (ibid, p.114). Kolers and Grunau (1976) did a phi experiment with two dots, one red and one green, flashing on and off. This gave the illusion of a red spot starting to move and changing colour to green mid-passage. Since the red dot is not moving and does not turn into a green dot we need to ask what is going on with this illusion. As the red dot moves we see it turn green as it moves towards its destination. The question is: how do we see the dot turn green before we actually see the green dot? One might think that the mind must first register the red dot and then the green dot and after that point the apparent motion must be played before the mind’s eye. To think this, way Dennett warns, is to demonstrate that one is still in the grip of the metaphor of the Cartesian Theatre (ibid p. 115).

To loosen the grip of this picture on our minds Dennett discusses two fictional processes which one could attribute to the brain. He calls them the Orwellian Process and the Stalinesque Process.  The Orwellian Process occurs when I misremember something because my brain tampers with what I remember so that I no longer remember accurately. The Stalinesque Process is where the brain projects a false picture of reality into the mind’s eye. Dennett notes that while a distinction between Orwellian and Stalinesque processes makes sense in the external world it is an illusion to assume that it makes sense as an explanation of what is going on at the level of the brain.

Let us think of both of these processes as they apply to the case of colour phi.  In the Orwellian case we did not see the apparent motion; our brain merely revised our memory and informed us that we did see the motion. In the Stalinesque case we unconsciously registered the two dots and afterwards our brain created a kind of mock event for us to watch. Dennett notes that once we give up the notion of Cartesian Materialism, we will see that there is no answer to the question of whether the Orwellian or Stalinesque process took place. He puts things as follows:

So here is the rub: We have two different models of what happens to the color phi phenomenon. One posits a Stalinesque “filling in” on the upward, pre-experiential path, and the other posits an Orwellian “memory revision” on the downward, post-experiential path, and both of them are consistent with whatever the subject says or thinks or remembers…Both models can deftly account for all the data-not just the data we already have, but the data we can imagine getting in the future (ibid, pp. 123-124)

So there is no fact of the matter which can decide between the two different stories. Dennett argues that the reason that we cannot decide between the two accounts is that there is really only a verbal difference between them. With Dennett’s rejection of Cartesian Materialism and his alternative multiple-drafts theory of consciousness we can no longer draw a non-arbitrary line to decide when an event becomes conscious.  There is therefore no fact of the matter as to whether an Orwellian or a Stalinesque process took place.

When Dennett claims that we cannot decide between the Stalinesque and Orwellian alternatives we are left with what seems like a mystery. In the external world a red object is not really moving and turning into a green object, yet Dennett is also denying that a Stalinesque show trial is played before the mind’s eye.  So the obvious question is: where does the movement of the ball occur? Dennett’s answer is that the ball does not move and turn green it only seems to. However, to say that a ball seems to move is to say that people have an experience of the ball moving. And this leads us back to our original question: what generates this experience, and how is it generated? Dennett thinks that this is a bad question because the brain does not need to create an experience of the ball moving; it merely has to form a judgment that such movement occurred:

The Multiple Drafts model agrees with Goodman that retrospectively the brain creates the content (the judgment) that there was intervening motion, and that this content is then available to govern activity and leave its mark on memory. But the Multiple Drafts model goes on to claim that the brain does not bother “filling in” the blanks. That would be a waste of time and (shall we say?) paint. The judgment is already in, so the brain can get on with other tasks. (ibid, p. 129)

This claim of Dennett’s is extremely strange. He is claiming that the brain judges that the motion occurred. However, as a matter of fact, we do not experiencethe motion; we only think we do.  The obvious reply to this is to categorically state that I do experience the movement and I judge that the movement occurred based on this experience. In other words, the experience is prior to the judgment. The experience is not of a fact in the external world (where no movement occurred); it is rather an experience of a person’s subjective qualia. When Dennett denies that it is the experience that leads to the judgment, he is leaving the phenomenal experience out and is focusing entirely on access consciousness.

The claim that Dennett is denying the existence of phenomenal consciousness is on the face of it an incredible claim. So before proceeding it is important that we show that this is indeed Dennett’s position. To demonstrate that this is indeed Dennett’s position I will now provide detailed quotes from him to make clear his position.  When discussing phenomenal space Dennett makes the following claim:

Now what is phenomenal space? Is it a physical space inside the brain? Is it the onstage space in the theatre of consciousness located in the brain? Not literally. But metaphorically? In the previous chapter we saw a way of making sense of such metaphorical spaces, in the example of the “mental images” that Shakey manipulated. In a strict but metaphorical sense, Shakey drew shapes in space, paid attention to particular points in that space. But the space was only a logical space. It was like the space of Sherlock Holmes’s London, a space of a fictional world, but a fictional world systematically anchored to actual physical events going on in the ordinary space of Shakey’s “brain”. If we took Shakey’s utterances as expressions of his “beliefs”, then we could say that it was a space Shakey believed in, but that did not make it real, any more than someone’s belief in Feenoman would make Feenoman real. Both are merely intentional objects. (Ibid, pp. 130-131)

The above passage is very instructive. It speaks to our topic of mental images and again shows that Dennett thinks of them as theorists’ fictions. Furthermore, his invoking of Shakey, who despite its verbal reports is not experiencing any items in phenomenal space, shows that Dennett thinks that we, like Shakey, despite our verbal reports are not experiencing anything in phenomenal space. Dennett is claiming that our brains may tell us that we have such and such experiences, and as a result of this brain report we form the judgment that we saw a red light move and turn into a green light. However, this judgment, despite appearances, is not grounded in a phenomenal experience.

It is worth noting that a lot of thinkers misinterpret Dennett’s claims on ‘seeming’ and colour-phi as indicating that he denies that we experience colours. This is not the case. Dennett’s arguments above only apply to colour hallucinations, Dennett tells a different story about how we perceive colour in the external world.

To understand Dennett’s views on colours it is helpful to think in terms of the primary quality distinction. One of the central motivations for claiming that the world is in your head is the existence of secondary qualities. When one looks at a beautiful garden one sees a variety of different colours: like the bright yellow sun-flower, the green-grass, the multi-coloured butterflies, the blue-sky and the bright-yellow-sun. Since the seventeenth century people like Galileo and Locke have been telling us that colours do not exist in the mind independent world. Colours are effects of light reflecting off objects and hitting our retinas in a variety of different ways. The majority of scientists since Galileo accept this dichotomy between primary and secondary qualities. Primary qualities are: Solidity, Extension, Motion, Number and Figure, while the Secondary qualities are: Colour, Taste, Smell, and heard Sounds. One consequence of accepting this picture is that the world is not as it reveals itself to us in our experience furthermore colours do not exist in a mind independent world. A further consequence is that we have a rich world which we experience consisting of taste, smells and colours but this world exists only within our minds. So on this view we have a subject, who is presented with certain experiences, and only some of those experiences correspond with a mind independent entity. The Cartesian materialist who accepts this world picture has a difficult job on his hands. Nowhere in the brain is the experience of a blue sky located or a yellow daffodil located. He may be able to provide neural correlates for these experiences but he will not be able to point to the spatio-temporal location where the experience is and the subject is located. So presumably the Cartesian materialist will have to argue for a strong emergence thesis.

Rather than going down this road Dennett interprets the dichotomy between primary and secondary qualities differently than most contemporary theorists. Dennett has discussed the status of colours throughout his philosophical development: in particular in his 1968 Content and Consciousness, 1991 Consciousness Explained and in his 2005 Sweet Dreams: Philosophical Objections to a Science of Consciousness. I will now try to give a synoptic view of Dennett’s views on the topic of colours. In his first major discussion of colours he noted that while most believed that colours are secondary qualities and do not exist in the external world there are reasons to doubt this conclusion.

He centres his criticism in terms of language and what we are referring to when we use colour words. If we view colours as secondary qualities we are committed to the view that when I refer to something red I am referring to something within my mind. Now if we accept this view then when two people claim that they are referring to something red then they we don’t know whether they are referring to the same thing, as their inner experience of red may be different and, we cannot decide because we would have nothing public to compare their experiences to. Now if we do not want to admit the possibility that a teacher can never know when his pupil has actually learned the meaning of the word ‘red’ we must admit that the reference of colour words is to public observable entities.

One difficulty is that if one accepts the solution to the sceptical problem of colour reference by arguing that words refer to publically observable entities is that it leaves us with a conundrum of where we say that colours exist. They don’t exist in the mind independent world, and they don’t exist in the mind, and there is nowhere else to exist. So one is lead to the silly conclusion that colours do not exist anywhere. This conclusion must be wrong; and Dennett correctly notes that colour words refer to publically observable entities that one can be right or wrong about (Content and Consciousness, p. 161). So they seem to exist and seem to exist in a publically observable sphere.

For Dennett since colours are publically observable entities which we can be right or wrong about then they must be a property of the external world. This leaves Dennett with the question: what property exactly are they? He notes that colours are not reflective properties of surfaces which can be cashed out at a sub-atomic level. This is because:

“Although the sub-atomic characteristics of surfaces that reflect light predominantly of one wavelength can now be described in some detail, those different types of surface do not correspond neatly to the colours we observe things to be.” (ibid, p. 161)

Also different wavelengths of reflected light can cause the same colour experience in a person. So the job of characterising what property colours actually are is more complex than one might assume. Dennett notes that when a person is referring to red we will need to cash-out what property they are referring to in terms like: The person is referring to the reflexive property of x or y or z…(and the disjunction associated with one colour might be very long).

Dennett asks us: if the disjunction of properties which can be associated with a person’s experience of colour have little in common with each other, are we driven to the conclusion that colours do not exist? To think through this possibility considers colour blind people who have poorer discriminative capacities than us and a hypothetical alien who has colour discriminative capacities which are greater than ours. He notes that we would say that the colour blind man who may say that ripe (red for us) apples and grass are the same colour is suffering from a cognitive illusion. On the other hand if an alien had greater discriminative capacities than us so that it would constantly see things as changing colour, we would also say that he was experiencing colour illusions. This is because the meaning of colour terms is defined in terms of OUR discriminative capacities; which means that WE judge certain things in the world to be red, green etc. So relative to our form-of-life these other people would be suffering from a form of cognitive illusion.

Dennett concludes with the following statement:

Colour is not a primary physical property like mass, nor is it a complex of primary properties, a structural feature of surfaces. Nor again is it a private ‘phenomenal’ quality or an ‘emergent’ quality of certain internal states. Colours are what might be called functional properties. A thing is red if and only if when it is viewed under normal conditions by normal human observers it looks red to them, which only means: they are demonstrably non-eccentric users of colour words and they say, sincerely, that the thing looks red. There saying this does not hinge on their perusal of an internal quality, but on their perception of the object, their becoming aware that the thing is red (ibid, p.163)

I am not really sure whether Dennett really manages to avoid the problem of where the experience of red is located. However it should be obvious that he is not denying that colours exist, rather he is claiming that they are not paraded in a Cartesian Theatre.

My reaction to Dennett’s views on hallucinations and mental imagery are conflicted. On the one hand I find myself arguing that when I call a mental image to my mind’s eye I do damn well experience a mental image I don’t merely SEEM to experience it. On the other hand I find myself worrying that this reaction of mine is simply me giving my judgments about my subjective experiences papal infallibility, I simply judge that because it seems to me that x is presented before the mind’s eye then I cannot doubt that this is the case. In the case of perception of an object in the external world I can make judgements which can be intersubjectively tested and these tests can either weaken or strengthen my claim. Or even if there are no other people around who I can test my perceptions by seeing if what I am seeing is an illusion, by inspecting the conditions of viewing whether there is good light, I can walk up to the object, walk around it, touch it etc. In the case of hallucinations such intersubjective, bimodal tests are not possible. Likewise in the case of mental imagery I cannot test the lighting conditions or touch the images or walk around them etc. So it is extremely odd that I would accord myself papal infallibility in the case of hallucinations and mental imagery given that I cannot really test my judgements[3]. All I can really appeal to is the fact that I intuitively know I experience mental imagery. However if the last three hundred years of science have taught us anything it is that we cannot uncritically be overly reliant our supposed certain intuitive judgements (nor should we ignore them). So I go back and forth on this issue and have never really solved it. Tom Clark feels certain that REAL SEEMINGS exist. Matt Bush argues that they don’t they are just a product of the manifest image. I have never really made up my mind on the matter. Recently Matt has informed me he may have an argument for the existence of conscious experience that does not rely on the myth of the given. Perhaps this argument will solve a lot of these difficulties. However I think my next project is to try and discover if ‘The Given’ really is a myth.


[1]This section on Real Seeming is taken from my much longer and more detailed take on Dennett’s view on Consciousness and Mind ‘Dennett and The Typical Mind Fallacy’. A reader interested in understanding in more detail how I understand Dennett’s view and where my view differs from his should check out that paper. They will find the paper in my blog and in my Academia.edu page.

[2]See Kolers, P. A and Von Grunau (1985) “Shape and Color in Apparent Motion” Vision Research 16 pp329-335.

[3]I discuss the rotation tests and neuroscientific tests of Kosslyn et al. In my ‘Dennett and The Typical Mind Fallacy’ but those third person tests are not the reason people believe they have imagery, rather their purported experiences are the reasons.



  1. Kanonymous

    Oh the mental masturbation of philosophers, how it leads to such ridiculous conclusions as Dennet’s.

    1. surtymind Post author

      Care to give more detail in your criticism? What is wrong with Dennett’s argument in your view? What is your view on the nature of hallucinations?


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