Leibniz Law, Brains in a Vat, and Hallucinations


            Dennett’s theory of consciousness which he has developed over forty five years has had to try and explain away various different phenomena which seem to indicate that we have experiences which do not occur anywhere. The phenomena of dreams, after images, mental images, illusions and hallucinations seem to be experiences which exist nowhere; they can be correlated with brain states but they are not identical with brain states. The experiences are not located in the brain or in the world for these reasons they are considered problematic for a naturalistic theory of consciousness. In this blog I will consider the phenomena of hallucinations and their relation to Dennett’s Theory of Consciousness.

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)

Naturalist philosopher Tom Clark (Naturalism.org) used a hallucination of music produced by the same brain area that perception of music is produced in 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 in 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.

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 as Tom did.

In his prelude to ‘Consciousness Explained’ Dennett discussed the nature of hallucinations along with the likelihood of the brain-in-the-vat scenario. Dennett notes that while it may be possible to stimulate a brain to think that it is hearing music and to feel that it is lying on a beach, it is far far beyond current and possibly all future technology to be able to manipulate a brain into thinking that it is a person walking around in the world, exploring it and making free choices about it. The reason Dennett thinks that such a brain-in-a-vat scenario is extremely unlikely is because there would be a combinatorial explosion involved in giving the brain in the vat exploratory powers in its virtual environment.

Given that the brain is a computational machine and that people report having very vivid hallucinations one could claim that Dennett’s arguments about the brain-in-the-vat scenario are contradicted by the existence of vivid hallucinations.  After all, since hallucinations do not exist in the external world but rather exist entirely within the brain then vivid hallucinations are a variant of the brain-in-the-vat scenario. So a critic of Dennett’s could argue that it is only a matter of time till we can develop technology which can construct brain-in-the-vat type scenarios using similar computational processes that the brain uses when constructing vivid hallucinations.

Dennett’s solution to an argument like the above one is to claim that that while some hallucinations are possible, strong hallucinations are impossible. By a strong hallucination he means:

“By a strong hallucination I mean a hallucination of an apparently concrete and persisting three-dimensional object in the real world- as contrasted to flashes, geometric distortions, auras, afterimages, fleeting phantom-limb experiences, and other anomalous sensations. A strong hallucination would be, say, a ghost that talked 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 it looked like” (Consciousness Explained: p. 7)

Dennett argues that for hallucinations to support a brain-in-the-vat scenario the hallucinations would have to be a strong hallucination of the type he detailed above. However he argued that strong hallucinations do not in-fact ever occur so obviously they provide no support for a brain-in-the-vat type scenario. However while he does not think that strong hallucinations are possible he does think that multi-modal vivid hallucinations are possible. He argues that these hallucinations are made possible because of the fact that the minds epistemic curiosity is typically limited during hallucinations. He points out that people who have strong hallucinations typically note that they feel peculiarly passive when they experience hallucinations. They do not feel inclined to explore or engage with the things that they hallucinate. The mind in this scenario is forced to interpret along certain lines which means that the hallucination will not be properly tested. So on this view a hallucination is not a complex three dimensional mental object that can be inspected from various different angles. Rather it is an illusion which is maintained by restricting the exploratory paths that you will take. So a hallucination is to some degree like a card trick, it is not real magic, rather it is a trick that works because it focuses your attention in certain limited directions. The primary difference is that in the case of the card trick there is a person consciously tricking and manipulating us while in the case of hallucinating there is no trickster rather there is just a complex interaction competing areas of the brain trying to predict the phenomena of experience and competing with each other for control of the brain.

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 (unless of course we are eliminativists about people’s experiences).

I will now consider some of the cases of Hallucinations that Sacks discusses in his book ‘Hallucinations’ and try to evaluate whether they support or contradict some of the central claims that Dennett makes about hallucinations. In chapter 1 of his book on Hallucinations Sacks discussed Charles Bonnet Syndrome a condition where patients who suffer from loss of sight sometimes experience visual hallucinations. The syndrome is estimated to affect 15 percent of patients (See: Teunisse, R. et al. ‘Visual hallucinations in Psychologically Normal People’). One patient of Sack’s who suffered from Charles Bonnet Syndrome was called Zelda, she had a series of hallucinations (caused by poor blood flow in the occipital and temporal lobes) which were extremely vivid and lasted a long time:

As we drove away from the beauty parlour, I saw what looked like a teenage boy on the front hood of our car, leaning on his arms with his feet up in the air. He stayed there for about five minutes. Even when we turned he stayed on the hood of the car. As we pulled into the restaurant parking lot, he ascended into the air, up against the building, and stayed there until I got out of the car” (Hallucinations p. 17)

As we have seen above, Dennett makes the claim that typically people are passive when they experience hallucinations, and Zelda’s report above fits this pattern. However she later went on to mention that when she experienced hallucinations which she could not distinguish them from reality without testing, Sacks describes them as follows:

“Occasionally, looking in a mirror, she might see her own hair rising a foot above her head and have to check with her hand to make sure it was in its usual place (ibid p. 17)

So here we can see that Zelda’s report does not support Dennett’s conclusion that people who have hallucinations are unusually passive and can be counted on to only investigate in certain directions. Zelda had a hallucination which she doubted was real and she proceeded to test her experience by using another modality (touch). So there was nothing passive about her reaction to the hallucination. Nonetheless when she tested her hallucination she discovered that it was not real. So her hallucination was not what Dennett would call a strong hallucination. It would be better classed as a vivid hallucination restricted to a single modality.

Above I discussed a hallucination which Sacks had as a result of an ocular melanoma. In his book ‘Hallucinations’ Sacks described an even more dramatic hallucination which he experienced as a young man when taking twenty Artane pills (Artane pills are used to treat Parkinsons’ Disease). Sacks described the following dramatic experiences:

“So one Sunday morning, I counted out twenty pills, washed them down with a mouthful of water, and sat down to await the effect. Would the world be transformed, newborn, as Huxley had described it in ‘The Doors of Perception’, and as I myself had experienced with mescaline and LSD? Would there be waves of delicious, voluptuous feeling? Would there be anxiety, disorganisation, paranoia? I was prepared for all of these, but none of them occurred. I had dry mouth, large pupils, and found it difficult to read, but that was all. There were no psychic effects whatever-most disappointing I did not know exactly what I had expected, but I had expected something.

                   I was in the kitchen, putting on a kettle for tea, when I heard a knocking at my front door. It was my friends Jim and Kathy; they would often drop around on a Sunday morning. “Come in, door’s open,” I called out, and as they settled themselves in the living room, I asked “How do you like your eggs?”  Jim liked them sunny side up, he said. Kathy preferred them over easy. We chatted away while I sizzled their ham and eggs-there were low swinging doors between the kitchen and the living room, so we could hear each other easily. Then five minutes later I shouted “Everything’s ready” put their ham and eggs on a tray, walked into the living room- and found it completely empty. No Jim, no Kathy, no sign that they had ever been there. I was so staggered I almost dropped the tray.” (ibid: p. 107)

Sack’s report is interesting for a number of different reasons. Firstly his hallucination occurred in two modalities both visual and auditory and both modalities were perfectly synchronised. Secondly his hallucinations were indistinguishable from reality for him. It is true that he did not test the hallucinations to see if they were real but this is hardly surprising as he had no reason to think they were hallucinations they were presented to him as being as real as any aspect of reality.

However clearly Sack’s hallucinations are not strong enough to count as what Dennett calls a strong hallucination. Recall that Dennett defined strong hallucinations, ‘as a ghost that talked 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’ (Consciousness Explained P. 7). Sack’s hallucination meets some of Dennett’s criterion the people he saw and talked to in his hallucination meet the criterion of a ghost who talks back to you. However the hallucination was in no way tactile and did not cast shadows. Unfortunately because Sacks did not realise he was hallucinating he did not test his hallucinations by touching them and inspecting them from a variety of different angles.

Later in his Hallucinations Sacks describes an even more vivid hallucination which a patient of his who suffered from a left hemianopia had.

“He has not recovered vision and retains a left hemianopia. He has, however, little awareness of his visual loss as his brain appears to fill in the missing parts. Interestingly, though, his visual hallucinations/filling in always seem to be context sensitive or consistent. In other words, if he is walking in a rural setting, he can be aware of bushes and trees or distant buildings in his left visual field, which when he turns to engage his right side, he discovers are not really there. The hallucinations do, however, seem to be filled in seamlessly with his ordinary vision. If he is at his kitchen bench, he “sees” the entire bench, even to the extent of perceiving a certain bowl or plate within the left side of his vision-but which on turning disappear, because they were never really there. Yet he definitely sees a whole bench, with no clear separation between parts composed of hallucination and true perception.” (ibid p.177)

In the above case the patients hallucination actively fills up aspects of experience that the patient is blind to. This hallucination while vivid is not a strong hallucination as it does not involve multiple modalities.

 Our brief look at some of the hallucinations which Sacks reported provided limited evidence that strong hallucinations occur. Though they do indicate hallucinatory experiences which are stronger than those predicted on Dennett’s model.  They do provide plenty of evidence for the existence vivid hallucinations which do not involve the subject taking a passive attitude to their experiences. So Sack’s descriptions of hallucinations do challenge some of Dennett’s central claims about hallucinations. However since Sacks was not directly addressing Dennett’s theory of hallucinations a lot of his descriptions do not directly connect with Dennett’s theory about hallucinations. To this end I constructed a short questionnaire designed to test whether people who experienced hallucinations of various different types did so in a way which conforms to Dennett’s theory.

It will take a couple of months till I get all of the data back from my questionnaire and can organise the data in a scientific manner. So I will discuss the questionnaire in detail in a later blog. However, since the first two people who answered my questionnaire have provided answers which contradict Dennett’s views on the nature of hallucinations I will discuss them here. It should be noted though that this discussion is entirely preliminary and I will provide much more detail in my later blog.

The questionnaire was structured as follows:

(1)Have you ever experienced a hallucination?

(2) Do you know what the cause of your hallucination was? E.g. Was it caused by the use of Drugs (either prescribed by a doctor or taken recreationally), or by epileptic seizures, or by nightmares, or blindness, mental illness etc? If there was some other cause of the hallucination beyond the ones I mentioned please state them? If you do not know the cause of the hallucination please state this.

(3) Did your hallucination occur in only one modality? E.g. was the hallucination primarily visual or did it also occur in other modalities? Did you hear your hallucination? Could you touch it? Or taste it? Or smell it? How many modalities did your hallucination manifest itself in?

(4) A strong hallucination is one “that talks back, that permits you to touch it, that resists with a sense of solidity, that casts a shadow, that is visible from any angle so that you might walk around it and see what its back looks like” (Dennett 1991 p. 7).  Would you describe your hallucination as a strong hallucination as defined above?

(5) When you experienced your hallucination did you experience either of the following attitudes (A) Did you feel the urge to stand passively and marvel at the hallucination? Or (B) did you feel like interacting with and inspecting and questioning the hallucination?

(6) When you experienced your hallucination did you (A) Passively marvel and stare at it? Or (B) Did you interact with it, and inspect and question the hallucination?

(7) How would you compare the vividness of your hallucination with the following experiences (A) Was the hallucination less vivid than a mental image? To test this call to mind the mental image of a friend’s face or a friend’s voice, was the image more or less vivid than your hallucination? (B) Was the hallucination less or more vivid than an after image? To test this look at a light bulb that is on for a few seconds when you look away from the bulb you should see an after image of the light in your field of vision. Compare the vividness of the after image with the vividness of your hallucination. (C) Was the Hallucination more or less vivid than Hypnagogic Images? (Hypnagogic Images are the imagery people sometimes experience when in-between being asleep and awake). If you cannot experience mental imagery, after images, or hypnagogic imagery you do not have to answer this section; though if you have experienced any of the above please try to answer the relevant questions.

(8) Describe your most vivid hallucination in as much detail as possible including the moments proceeding the hallucination and the moments immediately after the events.

The first two people to answer the questionnaire experienced hallucinations for different reasons. The first person who we will give the pseudonym Mary had hallucinations as a result of night terrors. In question 4 of my questionnaire I asked whether she had ever experienced strong hallucinations in the sense defined by Dennett. Here is her answer:

“Yes on several occasions – once I woke up from a nap on the couch and saw a huge black spider under the chair across the room. It was moving but staying under the chair. This happened while I was a don at University College in Toronto – I thought it might be a tropical spider come in with a load of bananas. I got up, upended the wastebasket so I could contain the spider and took a poke at it with a small wooden stick I took from under a plant pot on the table. As I was poking at the spider to prod it into the wastebasket, it dissolved into nothing. I went back to bed. I even thought it might be a dream until I noticed the waste basket and the stick were still there in the morning.”

Strictly speaking the above hallucination is not a strong hallucination in Dennett’s sense, however it multi-modal and it involves her interacting with her hallucination instead of merely passively staring at it. So it does to some degree contradict Dennett’s claims about the nature of hallucinations.

In response to question 6 Mary answered as follows:

“No, my first reaction is that it is real and often I am terrified – strange writhing geometric shapes descending on me while I lie in bed make me scream. Once I woke up (this was just a few years ago) and saw dark smoke billowing into the bedroom door – I assumed there was a fire downstairs and screamed and raced down the stairs to find the fire extinguisher… the smoke had completely vanished by the time I reached the dining room. This happened just about ten years ago. My husband was awakened too and following me down the stairs, poured us both a small brandy and we went back to bed. (B) I am usually either screaming or taking action. Once I saw a brilliant ball of light in the middle of the room, like a fire but white. I was paralyzed with fear and could not even scream… I was about 6 years old. I could see, vaguely, hands and a human form inside the light. I could see the light reflected on the tree branches waving outside my window. I had a puppy in the bed with me, and held him close to me. I was terrified. Then a voice said “believe” and I said “okay” and suddenly I went back to sleep. In the morning, when I woke up, I was still in the same position and the puppy was still asleep in my arms. I am an atheist, but because of this experience I can see why people believe in angels and burning bushes etc.”

Here we can see that Mary’s description of her hallucinations directly contradict Dennett’s claims about the nature of hallucinations. Mary does not merely passively stare at the hallucination but actively interacts with them.

The person who answered my questionnaire we will give the pseudonym John. John experienced his hallucinations after taking Lysergic Acid Diethylamide. He described one of his hallucinations as follows:

“I was sitting behind the desk at the door next to a nightclub venue I was a frequent visitor at (even though the night was one I hadn’t been to). The moments preceding I was looking around and people knew I was tripping hard; the venue was packed with people I knew well as a DJ and people kept coming over to check on me/marvel, mostly because it was my first trip and I was up and down a lot. I turned around and noticed that there were embossed raised patterns moving over the walls which matched the colour of the walls but had shadows and light qualities that made them look very solid and three dimensional; as above. I spent some time (I can’t remember how long) interacting with it, asking another friend who was tripping if they’d experienced anything like this and describing what I was seeing. Afterwards I got bored and turned back to the door and tried to talk to my friends, who were insisting that I didn’t sound like I was on acid but that it was very clear that I was. Their faces had a morphing quality and my vision was either panoramic or reminiscent of a fish-eyed camera. I experienced constant visual hallucinations and a few whole-experience hallucinations (having a strong sense of everything looking drawn in the style of Invader Zim, colours and stuff when I closed my eyes, time freezing to music, the experience of total innocence and an epiphany that I should stop experimenting with drugs on a weekly basis if I was going to get anywhere in life — I did, but probably because the intensity of the trip freaked me out). It was very difficult to choose one hallucination because each time I’ve dropped acid I’ve had extremely vivid experiences which I’ve been aware of because they’re so unlike reality. The first time I dropped I had the most full on experiences. I went with this hallucination rather than others because it was close enough to the real world to be able to describe confidently and in detail — I can actually relate it enough to the real world. Much of it was mental — the way I was thinking was completely different. In retrospect I’d say that it felt thoughts were moving as a Lorentz attractor.”

John’s experience was of patterns which appeared to cast shadows and he claimed that he interacted with the patterns so they have some of the qualities of a strong hallucination. John noted that he was able to touch the pattern that it cast a shadow and could be viewed from a variety of different angles. So John’s experiences were of a type that Dennett claims do not occur.

This brief discussion indicates that Dennett’s claim that people cannot experience strong hallucinations may be false. I will go into give a systematic analysis of the details of my questionnaire in my next blog. Here I merely want to note that Dennett’s claims that people cannot experience strong hallucinations and that they passively stare at hallucinations is not supported by the first two people who answered my questionnaire nor by the reports of some of Oliver Sacks patients. This raises difficulties for Dennett’s view on the nature of hallucinations as they seem to have a quality of reality for subjects that Dennett’s theory would predict.

One could argue that while hallucinations are correlated with brain states they are obviously not identical with them. If I have hallucination of a person who I am  talking to and inspect from different angles, this hallucination not being played out within an internal Cartesian Theatre, it is not identical with the grey matter in my brain, and it does not actually occur in the external world. So if such a hallucinations do indeed exist then, as people’s report indicate they do then we need a way of explaining where these hallucinations exist. Dennett’s solution is to argue that it only SEEMS to us that we are experiencing these hallucinations and mental Images; we are not actually experiencing them. He thinks that the only other solution is to assume that hallucinations are “Real Seeming” painted in Cartesian theatre by some kind of ghostly substance a position he obviously wants to argue against as i. 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.

A problem for Dennett’s position is that while it is somewhat plausible to claim that people don’t actually experience mental imagery or brief illusions it is intuitively less plausible to argue that strong hallucinations are not real seemings. Multimodal hallucinations which we can inspect seem on the face of it to be less easily explained away as mere seemings. It is for this reason that Dennett finds it necessary to argue that strong hallucinations do not occur, and that when less vivid hallucinations do occur Dennett argues that they result in the passive attitude we take towards them. As we have just seen the verbal reports of the people I have questioned and of Sack’s patients do not support Dennett’s claims about the nature of hallucinations. In my next blog I will go into this in more detail. For now it is sufficient to note the discrepancy between people’s reports and Dennett’s claims. So since Dennett has not proven that hallucinations have the structure which he argues for in ‘Consciousness Explained’ all he really has left is his claim that hallucinations are mere seemings. Now his claim that hallucinations are mere seemings could indeed be true, however the only real evidence to support it is that it helps Dennett avoid breaking Leibniz’s law when explaining the nature of hallucinations, mental imagery etc.

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 tests are not possible, though bimodal tests are. 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, hallucinations, illusions etc[4]. 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 but I lean towards Matt’s view.


[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.

[4][4]I should add that I personally have only ever experienced weak mental imagery, I have also experienced illusions like colour phi but I have never experienced any hallucinations myself even under the influence of LSD.


One thought on “Leibniz Law, Brains in a Vat, and Hallucinations

  1. Ashok

    this enquiry is profound.. it penetrates in to the domain of cognition, and I the long run the merging of language and cognition where the auditory images, or sound images have an autonomy of its won, neither in the world nor in the brain events; yet peculiarly in the domain of anticipatory perception, where the instinctual emotional takes a back seat…


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