Why life was better in London the 18th century than in the 17th century

I just finished to listen to two audiobooks: The English Society in the Eighteenth Century (Roy Porter), and Restoration London: Everyday Life in the 1660s. Both books where equaly entertaining and interesting. Now I can safely say that I would have prefer to live in the 18th century London than in the 17th century london. In the 17th century, you was not really free to think and express what you wanted. In the 18th century, things started to change. There was a lot of freedom of faith and even if people still expected you to believe in god, you was not necessarily burned anymore for being an atheist. Also, trade freed many poeple from their usual role. In the 17th century, you was either a poor paysan or a landowner. In the 18th century, you was either a poor paysan, a poor businessman, a middleclass buisinessman, a rich businessman or a landowner. I would be curious to read about London in the 19th century...

Book Review: Sight Unseen (part III). Are frogs conscious?

As I indicated in part II, although she has no conscious experience of the presence of a pencil, Dee Fletcher is able to catch a pencil using the same hand orientation as would a person with unimpaired vision.

As it turns out, some people have brain damages that cause the opposite effect: They can see and describe a pencil (they are consciously aware of the presence of a pencil) but they are unable to make the necessary movements to grasp it (Balint's syndrome). What these people have is not a motor problem. Balint "...could deduce that from asking the patient to point to different parts of his own body using his right hand with his eyes closed: there was no problem."

From the analysis of Dee Fletcher's case, it seems that when we are reaching for a pencil, we think that we need to be aware of the presence of the pencil, but actually not. We do not need to be aware of its presence. Dee Fletcher is not aware of its presence, but she can catch it. Reaching for objects is an ability that has been hard wired in our brain far back in our evolutionary history and it did not require consciousness. For this kind of motor acts at least, we are not better than "zombies".

A further indication that it is indeed so comes from experiments on animals:

When frogs catch preys, they use a different brain module from the one that guides them around visual obstacles blocking their path. This has been demonstrated in the following experiment:

First of all, they are two things that you need to know about frogs:

1) frogs’ brains can regenerate new connections when damaged.

2) the right eye of a frog is connected to the left hemisphere of its brain (and vice versa).

In these experiments, the optic nerves that brought information from the right eye to the "prey catching part" of the brain were cut. A few weeks later, however, the cut nerves re-grew and connected with the "prey catching part" of the brain but on the wrong side of the brain. As a result, when these frogs were brought in presence of a pray on their right side, they tried to catch it on their left side. However, when brought in presence of an obstacle on their right side, the frog correctly avoided the obstacle (this because the "obstacle avoiding part" of their brain had not been rewired.

The "prey catching part" in these frogs was now wired up the wrong way around.

But this did not mean that their entire visual world was reversed. It was as though the frogs saw the world correctly when skirting around an obstacle, but saw the world mirror-imaged when snapping at prey. Hence, frogs do not experience a global visual world created for all purposes. Frogs have specialized parts of their brain using visual information independently to perform different tasks.

Does that mean that frogs do not experience a visual world at all? Maybe they do, but as a matter of fact, they do not need one for catching prey or avoiding obstacles. By the way, we also do not need one for catching objects or avoiding obstacles. But we do experience a global visual world. This conscious visual experience might only be present in animals needing to communicate and/or to plan ahead...

For a long time I thought that any animal with a memory was conscious. Maybe frogs are conscious to some extend but it seems that they can function without experiencing a visual world as we do. Their kind of consciousness, if it exists, might be very different from ours.

Book Review: Sight Unseen (M.goodale, A. Milner) Part II:Doing without seeing

Now you will be surprised! You remember that Dee was unable to see the shape of things? Well, as it turns out, if you show her a pencil, she is unable to say what it is, let alone if it is held vertically or horizontally. However, ask her to grasp it and she will grasp it just like anybody else! With the same "on the fly" right orienting of her hand and with the same "on the fly" narrowing of her thumb - index distance! Just like if she could see it!


Just as surprising: She can walk around in a landscape full of obstacles without ever stumbling on any! She avoids them all like you and me!


The authors presented her a mailbox which opening was rotatable so as to enable orienting it in any chosen direction. When Dee was asked to hold an envelope and to rotate the envelope in such a way as to match with the opening of the mailbox: She failed lamentably. She seemed to orient her envelop randomly. But when asked to post the envelop through the slit: she succeeded easily, orienting correctly her hand just like you and me!


The authors asked her to grasp various objects while captors where posed at various parts of her hand so as to be able to analyze finely her moves. She grasps things in the exact same way as anybody else: adapting the orientation of her hand on the fly and adjusting the thumb-index distance to  the size of the object! Objects that she cannot "see" and which dimensions she is unable to guess!


As it turns out, Dee has the part of her vision controlling her actions intact but has the part of her vision constructing her perceptual representations damaged. She is living evidence that part of our vision driven actions are unconscious. We do not need to be aware of the orientation of a slit to be able to slide something in it. However, we do need to be aware of the orientation of said slit to be able to describe it!


We have many independent visually controlled processes in our brain. Many of them (grasping things, walking around avoid things, ...) still work even when, like Dee, we have lost the ability to consciously see these things!


In my next post, you will see how some people have the exact opposite problem than Dee.

Book Review: Sight Unseen (M.A. Goodale, A. D. Milner) Part I : A tragic accident

The undertitle of this book is "An exploration of concious and unconscious vision". This book relates some truly amazing facts about consciousness and vision.

The first chapter relates to a patient of the authors: Dee Fletcher. This woman had an accident. She got poisoned by carbon monoxide while taking a shower. As a result, she suffered some brain damage and lost part of her vision. Dee can still see colours but she cannot recognize her mother. She can see the texture of things but not their overal shape. She can see the hairs on a hand's skin but she cannot recognize a hand. She has vision...without shape. she can tell that something is made of red plastic or shiny metal but she has no idea of the shape of the object. Dee has difficulty to separate an object from its background;

The authors tested Dee scientifically, asking her to perform various controled tasks.

When presented with a line pattern, she could say that they were lines but could not say if they were horizontal or vertical.

When presented with a drawing of an apple, she could not recopy it; but when asked to draw an apple from memory, she did very well.

The rest of her mental live seems to be perfectly normal.

All this seems weird but the result of further tests is even much weirder! (To be continued)

Book Review: Consciousness Explained, (Danniel Dennett) (IX): Autostimulation

This part of Dennett's book attempts to explain the origin of "thinking". The hypothesis proposed is very interesting, very simple and convincing.


Consider a time when language just started to develop: hominids would have been grunting various sounds and start to attach meaning to them. Modern apes do that all the time. For a striking and well documented example, see vervet monkies's alarm calls for snakes, eagles and leopards.
We can speculate that at time, our early hominid would ask a question to his comrades by uttering something and could expect an answer (in a social group, "I scratch your back, you scratch my back" behaviours may be beneficial to the individual if  the group members have the necessary memory to keep track of other members behaviours). Our hominid ancestor could have developed the habit to ask questions and to answer other's questions (e.g. "are there crocodiles in this river?"). One day, one of our hominid ancestor might have asked a question while nobody was there to respond, but since he himself heard his own question, it triggers in himself the urge to reply to it! And to its delight, he found out that he just answered his own question! Asking oneself question could arise a side effect of asking other questions. Speaking to oneself could have an evolutionary advantage if the connection between the different parts of the brain was not optimal. E.g. a first part of the brain might need a piece of information present in another part of the brain, not efficiently connected (yet) to said first part. By broadcasting its request in the environment and by relying on a pair of ears to pick it up, said first part could establish a virtual link between both badly connected parts of the brain.

Talking out loud is one possibility, drawing pictures to oneself is another. One can also see how making such "communication with oneself" more private would be advantageous.


This scenario strikes me as a good candidate for how "thinking" evolved!

Book Review: When Money Dies (A. Fergusson)

I "read" this book as an audiobook. I found it interesting but not particularly exciting. It mostly relates to the economic life in Germany between the end of the First World War and c.a. 1925. During this period the Mark devaluated more than 1.000.000 times with may dramatic (and interesting consequences). What I will remember from this book is the following:

• Inflation (i.e. when your money looses purchasing power) pushes people to buy more things. Since their money looses value day by day, they try to convert it into goods as soon as possible. This is I think the reason behind the small controlled inflation that our government seems to tolerate and even promote. Inflation triggers increased money spending.
  Inflation decreases unemployment. Indeed, since people buy more stuffs, the industries produce more stuffs and we need more people to produce them.
  During inflation, it is a good idea to buy company shares. Companies are real stuffs and they do not lose value. Your money however does lose value. Stock shares increased in value a lot during this high inflation period.
  During inflation, almost everybody suffered a lot. The middle/upper class suffered a lot. People were not using doctors, lawyers, accountant and so on anymore. They were busier trying to find food.  The civil servants suffered a lot. They were not protected by syndicates and their wages did not increase or not much while everything was getting much more expensive. The renters suffered the most. Their rent was a fixed amount... The workers suffered relatively less because they were better organised to defend themselves. They could make strikes and ask for raises in their salaries. They also lost purchasing power during this period but less than the people above. The Farmers were the lucky one. Their product got more expensive and people still had to buy them. Also, they always had enough to eat due to their own production. Furthermore, they had to repay the big investments they did but their debt appeared tinier and tinier in relative terms. At the end however, the city people came to steal the food present in the farms...
• Deflation (when stuffs get cheaper every day) pushes people to keep their money because this money will permit to buy more tomorrow than today. This is something bad for "the economy" because people stop to buy stuffs. It also increases unemployment because since nobody buys anything anymore, industries do not need to produce so much either.

This inflation was apparently the result of the "high reparation" demands from France and England and from the fact that the German authorities never hesitated to print more and more money to cover their expenses, thereby devaluating it... 

Consciousness Explained (D. Dennett): Part VIII; The Baldwin effect

We are the result of natural selection. Natural selction works by 1) allowing mistakes in the genome of an animal's offspring and 2) selecting the offsprings whose genome makes them better at survival/reproducing. If you are not familiar with this concept, go to buy "the selfish gene" from R. Dawkins (a live changing book).

The Baldwin effect is a mechanism that speeds up natural selection in a very peculiar way. It require you to have a plastic nervous system, in short: an ability to learn. If there is a "good trick" out there for an animal to learn, e.g. a behaviour that would greatly enhance an animal success at reproducing, any animal having this good trick hard-wired in its genome would be at an advantage. However, since having this "good trick" within it would be an all or nothing story, there maybe lacking a sufficiently progressive slope for natural selection to opearte swiftly. Hence, the time for this trick to be hard-wired in every animal within its species might be very long indeed. See left side of the figure below:

However, species not having this good trick hard-wired in their genome but having a plastic brain wired in such a way that they are capable to learn that trick during their livetime, are able to evolve the hard-wirering of this trick much faster. Here is how it works: Animal A has a brain slightly more able to learn the good trick than animal B. Animal A has therefore an advantage over animal B and will therefore probably leave more offsprings. Amongst said offsprings, animal A', due to a fortunate mutation, is still beter at learning the trick than its parent was, A' acquires it faster in his livetime than his siblings and therefore outcompete them and makes more offsprings. Amongst the kids of animal A', animal A'' is gifted with a further fortunate mutation that makes him particularly able to learn the good trick...and so on and so forth until the good trick (or at least the ability to learn the trick damned-easy) is practically hard-wired in the genome. What the Baldwin effect provides is a gentle slope for natural selection to operate on (see right side of the figure above). Any small improvement in the learning ability of the individual gives him a higher chance to hit on the good trick(s).

This makes clear the big advantage that a plastic brain provides to a species.

Book Review: The Facebook Effect (D.Kirkpatrick)

I just finished listening to the corresponding audiobook. The book was really enjoyable and interesting. Key to the success of Facebook seems to be the very long term vision of his creator. The company was also very strategic in the way it developed. One example: Facebook decided to open at US campus already using another social network but when doing so, it also opened at all surrounding US campuses in order to create "pressure" from outside and to "gain" the campus for itself. The ambition of Facebook seems to become a kind of platform on the internet (just like windows is a platform). Another ambition of Facebook seems to provide to its member a kind of "identity card" that would become the standard identification method on the net. It also seems to ambitions establishing a currency. In a nutshell, it is a kind of worldwide government with extensive democratic control that Facebook aims for. His boss is depicted as being an idealist who does not put money as its priority at all. He is presented as being someone willing to bringing openness and transparency on the web.
Again, the book is very entertaining and I really enjoyed listening to it. It is motivating and it gives some ideas of what to expect in the future of internet.

Book Review: Consciousness Explained (D. Dennett): Part VI:The Precognitive Carousel

Dennett reports an experiment made by Grey Walter. It consisted in presenting a carousel projector for showing slides to patients having electrodes implanted in the motor cortex of their brain. The patients were given a controller with a button for them to push on when they wanted to switch from one slide to the next. Actually, the controller was a dummy controler and the slides where changed in function of the signal received by the electrodes. The results? The patients experienced that the slide projector was anticipating their decisions. The slides would change just before they had decided to change them. This again seems to point to consciousness arising after that the decision is made. Dennett however once again express his view that timing of conscious events is not possible. an opinion of him that I still need to study further before to accept it or to reject it.

I received an interesting message on Facebook from a reader (Jason Weber). I will probably post it as a note to chapter V.

Book Review: Consciousness Explained (D. Dennett): part V : Delay of consciousness

In the 80's, Libet reported an experiment where subjects (wearing scalp electrodes) were asked to flex one hand at the wrist while noting the position of a moving spot at the precise time they formed the intention to flex. Afterwards, said subjects reported where the spot was at the moment they decided to flex their hand. The electrical measurements performed by the electrodes permited to determine that the subjects flexed their wrist 350 to 400 ms before the time indicated by the subject as being his time of intention to flex.


This experiment seems to indicate that we are conscious of the decisions made by our brain AFTER they have been made.


It seems to me  that Dennett does not like this conclusion (although he does not express it explicitly) which seems to contradict the intuition that our conscious acts control our bodily motions.

He considers this kind of explanation as being stalinesque (i.e. presuming that the brain delays consciousness in order to be able to presents all the facts it wants to present in the order it wants them to be presented).


He points out that there is an alternative explanation: "the subjects were conscious of their intentions at an earlier moment, but this consciousness was wiped out of memory (or just revised) before they had a chance to recall it". He calls this kind of explanation "Orwellian" because history is re-writen after the facts.

Finally, he also points out that yet another explanation which is that "an element of content becomes conscious at some time t, not by entering some functionally defined and anatomically located system, but by changing state right where it is: by acquiring some property or by having the intensity of one of its properties boosted above some threshold."

Dennett seems to doubt very much the possibility to equal the time of reporting of the subject with the time of conscious experience.


Personally, I would call on Ockham's razor with such an issue: stick to the simplest hypothesis until proven otherwise. Hence, as far as I am concerned, this experiment seems to indicate that consciousness arises after the facts. Clinical cases like the case of Mrs. Dee in "sight unseen" are also pointing toward such an explanation.


I think Dennett doubly dislikes the results of this experiment. 1) as mentioned earlier, it places consciousness after the events and 2) it looks like we are watching a movie of our live in a cartesian theater (he despises the idea of the cartesian theater).


[this page has been amended on 12/03 after a reader pointed to me some errors of interpretation. Thanks to Jason]

Book Review: Consciousness Explained (Daniel C. Dennett): part IV (his model of consciousness)

The main purpose of Dennett's book is to 1) refute the carthesian theater model of consciousness (see image below), and 2) introduce his own new model instead. He calls his new model the multiple drafts model.


The Cartesian Theater:


A representation of the carthesian theater model of consciousness is depicted below:



In this picture, we can see the head of a man looking at an egg being fried in a frying pan. Inside this man's brain, a screen is present where the vision of the man is projected for a smaller man to look at. The smaller man (or homonculus) is a methaphor for a central part in the brain where "consciousness happen".
The carthesian theater model assumes that their is a central "organ" in the brain where what happens is what you are conscious of. Its most naive form is the idea that a spectator must be present in the brain in order for the senses inputs to be experienced. Descarte for instance considered the pineal gland to be such a center from where the input of the senses could be transduced from mechanical signals (Descarte was not aware of the electrical nature of these signals) to spiritual meanings for the soul's benefit.


The Multiple Draft Model:




In this model, there is no centre where everything has to converge in order to be experienced. On the contrary, different conscious events are generated at different places in the brain. Each element within an event is discriminated only once. For instance, if you see a cow, the brain discriminate the presence of a large object, then it notice it is an animal, then that it is a cow. The discrimination and its fixation in the memory is enough for the phenomenum to be conscious (if probed).
What will be conscious will depend of the time of the probing. If you probe too early, you will only experience the diffuse presence of a large object. If you probe very late, you will have forgotten everything.
There is no need for the discriminated element to be sent or linked to a central theater. The conscious experience will originate from the locus of the discrimination. Also, each element is constantly updated/modified (e.g. interpreted, refined, partly erased, ...) due to interaction with the rest of the brain (pre-existing memories, new inputs, ...).
The multiple draft model makes"writing it down" in memory criterial for consciousness.


This can be best understood by looking at the color phi phenomenom at the bottom of this post (I do not insert it here because it is a moving image that would disturb your reading). To most people, the color phi phenomenom is experienced as a red spot traveling from left to right and changing color midway to become green. However (and you can easily convince yourself thereof by hiding one of both spots), it is in fact nothing of the sort. It is simply two fixed spots, one red and one green, the first blinking out of phase with respect to the second.


When experiencing the color phi phenomenom, the brain does not need to place intermediate spots after having experienced the second spot because there is no part of the brain present to "watch" these intermediate spots, your brain (i.e. you) just "knows" that the spot moved.


When remembering past events, you don't feel them happening again, you just know they happended. For instance, if you try to remember with as much details as possible the pain you experienced the last time you knocked your toes against an obstacle, you will not experience real pain, you will just realize that you "know" what kind of pain it was (which intensity, which location...).


His model makes a lot of sense but what remains unclear to me is the following:


At any given time our brain is processing a lot of inputs: external inputs from our various senses and internal inputs. We clearly do not feel conscious of them all. Cerrtainly, we do not feel conscious of many of them "simultaneously". Dennett seems however to say that whenever an object of our phenomenology is discriminated by our brain, it is "conscious".


On another hand he alludes to the notion of probing. Probing that would determine what is reported as conscious. He seems to make a difference between what is conscious and what we report as conscious experience when asked (i.e. probed) to report.


What I don't get is what he exactly means by his notion of "probing". I get it only partialy. I get it when by probing he means triggering a report from the conscious subject: e.g. asking the guy what he sees/feel. However, we are not constantly being asked to report on our internal states and we feel nevertheless conscious in these apparently "non-probed" times.


Of course, I suspect that his notion of what the probing is is more subtle than that. I suppose that internal inputs and external inputs can themeselves serve as "probes".


Maybe this notion is better explained in the rest of the book.


Once very important phrase he wrote is "The multiple draft model makes"writing it down" in memory criterial for consciousness".


I fully agree with the fact that writing something down in memory is critical for consciousness!


I have my own partial theory on consciousness and I should maybe explain it here before to continue Dennett's book. In view of the bold sentence above, I suspect that his views and mine overlap largly.

Book Review: The Gnostic Gospels (E. Pagels)

Actually the title of this post should better read "Audiobook Review" because it is in this format that I "read" it. The book is about the secret live of Jesus and about the alternative forms of Christianity that were existing c.a. 50-150 A.D. 

The author of the book is a historian. The content of the book is an analysis of the so called "Gnostic gospels".

You might know that the gospels of Matthew, Marc, Luke and John where not the only gospels written in the period 100-400 A.D. Several others gospels (e.g. Mary, Thomas, Truth, Philip, Judas) also date from this period. They are authentic documents that were discarded by the church when compiling the New Testament.

Here are a few differences between the canonical version of the New Testament and some of these Gnostic gospels:

• Jesus was not resuscitated in flesh but in spirit only,
• The god of the ancient testament is not the same as the god of the new testament: the god of the ancient testament is "the creator", a kind of artisan who wrongly wanted to believe that he was the only god. Actually he had a "mother", i.e. the ultimate cause of everything and "father/mother" of Jesus,
• The Orthodoxs are praying the "creator" while the Gnostics are praying his "mother",
• The Christian god was a woman or had both sexes intermingled within him/her,

and so on...

In a nutshell, the author indicates that there was a conflict between an orthodox Catholicism which was in a process of strengthening itself and which was quite dogmatic, and a Gnostic Christianity which was rejecting the authority of the bishops and was much more open to an evolving and therefore changing Christianity.

This book gave me a better idea of how the church became what it is and why it resists so much the ordination of women for instance (the Gnostics were in favor of woman preachers)...

For those who wonder: No, it did not convert me to anything.

Book Review: Consciousness Explained (Daniel C. Dennet) (Part III: his method)

This is a continuation of part II

In his book, Dennett decides to rely only on what can be observed and recorded by an external observer of the conscious being. He does not wish to rely on introspection at all. Why this? Because he thinks (rightly to my opinion) that we are much less knowledgeable about our inner world than we think.

When asked, people will tend to think that their visual field is uniformly detailed and focused from the centre out to the boundaries. The following experience demonstrates otherwise:

Take a deck of playing cards and remove one card in such a way that you do not know what is on the card. Now close your eyes, turn the card so that it faces in your direction and place the card at the left periphery of your visual field. Now, when you will open your eyes, keep your focus on a point situated straight in front of you. Now open your eyes and, without stopping to stare at said point, try to guess which card it is. The amazing thing is that you cannot even say if it is a red or a black card! However, you can very well see that there is something there and if you move your hand a bit, you are totally seeing said something moving too. Now start slowly moving the card toward the centre of your visual field while keeping you gaze fixed right in front of you. You will be amazed to notice that the card really need to be damned close to the centre of your visual field before that you can even tell its colour or if it is a queen or king!

Book review: Consciousness Explained (Daniel C. Dennett) (part II: A party game called psychoanalysis)

I continue here my review started earlier.

This book is well worthreading can be obtained here.

  

3. A party game called psychoanalysis

Here, Dennett introduces us to a funny party game and argues that perception mights have the structure of this game.

The game works as follow:

1. Explain to the group that one person (the victim) will have to leave the group while another person (the dreamer) will have to explain his last dream to the rest of the group. Upon his return the dreamer will have to ask questions to guess what was the dream and who was the dreamer
2. Ask the victim to leave the room,
3. To the others, explain that the dreamer will have to answer all questions of the victim only by yes or no and according to a pre-defined scheme; for instance, the dreamer should answer "yes" if the question finishes with a vowel and "no" if it finishes with a consonent (or any other such arbitrary rule). Introduce also a rule, overruling the first one, imposing that the dreamer may not contradict himself,
4. Ask the victim to come back in the group and let him start asking his questions,
5. Once the victim "found" the dream (e.g. the dream was about a jaleous elephant with PZ Myers on its back entering the bedroom of the dreamer while he was playing monopoly with god) and guessed who had the dream, explain him how the dream was generated and explain him that said dream was actually purely the product of his (the victim) own brain and of a random process. Hence, joke that it is actually the reflection of the victim's own subconscient.

Dennett proposes the following parrallel between this game and the brain:

A part of the brain asks questions and these questions are answered on the basis of the data collected (by the eye for instance). By going back and forth in this process, the perception is refined, objects are identified, recognized, categorized.

For an hallucination to happen, all we need is to have the question asking part of the mind performing normally and to have a random or disorder or arbitrary sequence of yes and no as answers.

In the game, the questions asked by the victim's mind was supposed to reflect his current expectations, obsessions, worries,... Hallucinations are usually related in their content to the current concerns of the victim.

Hallucinations are the normal result of prolonged sensory deprivation. Indeed, the mind keep asking questions to the senses but get no answers, no input above the usual threshold for a nerve signal to be considered a valid input. As a result, the mind starts to lower the treshold until he gets some inputs. However, since there is no input at all but only the background noise, the mind need to lower the treshold so much that he ends up receiving said background noise. Hence, the brain recieves random answers to his questions.

These hallucinations start weak and grow stronger. This could also explain the origin of dreams...

What Freud´s theory of dreams says is that “something” in our mind is composing a dream for the benefits of our ego but disguises the true meaning of it. This, according to Dennett (and to me) is pure bullshit.

Book review: Consciousness Explained (Daniel C. Dennett) (part I: The brain in the vat)

Daniel C. Dennett is a proponent of consilience. He is trained as a philosopher and is the director of the center for cognitive studies at Tufts University. He is an interesting fellow because he is very knowledgable in sciences like biology or computer sciences and he applies his scientific knowledge and methods to the unraveling of the deepest mysteries such as ...in this book...consciouness. Not less!

I am only half through the book but I think that it can only do me good to summarize what I thought interesting so far.




Prelude: How are hallucinations possible?
1.The brain in the vat and 2. pranksters in the brain

If you have seen "The matrix", you know what is meant by "brain in the vat". Some philosophers have argued that it is not possible to tell whether we are really out there interacting with the real world or if we are just brains in a bocal filled with nutritive fluid and fed with inputs simulating a world.

Dennet expresses the view that we could be brains in a vat only if we were not given exploratory power. Give us exploratory power, even very limited, and the number of possible worlds that the vat master must generate for us to still believe in the illusion of a real life becomes enormous. Literally, it is a combinatorial explosion. In a nutshell, to get the feeling of the real world, you NEED the real world if you have exploratory power.

à we are not brains in vats

àstrong hallucinations are impossible (they are similar also simulations of a world we really believe in).

The credibility of an hallucination is inversely proportional to the strength of an hallucination.

Nevertheless, convincing, multimodal hallucinations are frequently experienced.

Dennet goes on to explain that triggering the optic nerve anywhere between the eyeball and the brain could produce an hallucination and that people having an hallucination are often very passive in the face of the hallucination.

To give the person the illusion of being active, the "illusionist" (e.g. the vat master) must know in advance the exploratory intentions and decisions of the victim or induce them.

Here stop my review of the beginning of Dennett's book. The following interesting guy seems to think that strong hallucinations are commonplace: