Monday, March 5, 2018

Thoughts on Jaynes's Breakdown of the Bicameral Mind

It is one of those books that is either complete rubbish or a work of consummate genius, nothing in between!  Probably the former, but I'm hedging my bets.
— comment about Jaynes's The Origin of Consciousness in the Breakdown of the Bicameral Mind in Richard Dawkins's The God Delusion, 2006.

I've just read Julian Jaynes's 1976 book The Origin of Consciousness in the Breakdown of the Bicameral Mind, and here I'm posting my thoughts; built on roughly the structure of, though wider-ranging than, a book review.

This book engages three of my particular interests, deeply entangled in the instance so that they come as a package.  I'm interested in the evolution and nature of the human mind, which of course is Jaynes's subject matter.  I'm also interested in how to read a forceful presentation of a theory without missing its fault lines.  And I'm interested in how best to present an unorthodox theory.  (I've touched on all three of these in various past posts on this blog.)

To be clear:  I enjoyed reading Jaynes's book; I think he's glimpsing something real though it might not be quite what he thinks it is; and I think his book, and his ideas, are worth studying.  Keep those things in mind, moving forward through this post.  My interests will cause me to emphasize criticisms of Jaynes's theories, I'll be trying to assemble a coherent alternative to contrast with Jaynes's theories, and with all that going on in this post the positive aspects of my assessment might get a bit buried.  But I wouldn't be paying such close attention to Jaynes if I didn't see his work as fundamentally deserving of that attention.

When studying any forceful presentation of a theory, there is risk of joining the author in whatever traps of thinking they're caught in.  The best time to scout out where the traps/fault lines are (take your pick of metaphors) is on first reading.  That's true of both orthodox and unorthodox theories, btw, indeed it's a common challenge for orthodox theories, where the traps must be easily overlooked for the theory to have achieved orthodoxy.  Unorthodox theories are sometimes presented with markers that make them sound crazy, in which case the larger challenge may be to avoid underestimating them; but a strong unorthodox presentation, without craziness markers, — such as Jaynes's — can also contain hidden traps, and moreover the reader has to distinguish genuine traps from, so to speak, legitimate unorthodoxy.

Hence my reading Jaynes slowly and cautiously, jotting down whatever notes came to mind as I went along.

It wouldn't be difficult for Jaynes's basic theory to sound crazy; Dawkins has a point there.  At its baldest, Jaynes's big idea is that until about four thousand years ago, human beings didn't have a conscious mind, but instead had a self-unaware left brain that took orders from hallucinated gods generated by their right brain — the bicameral mind.  You don't want to dive right into the thick of a thing like that, and Jaynes doesn't do so.  He builds his case slowly, so that as he adds pieces to the puzzle it's clear how they fit.

I see no need to choose between completely accepting or completely rejecting Jaynes's ideas, though.  There seems room for Jaynes to be seeing some things others have missed, while missing some factors that lead him to a more-extreme-than-necessary explanation of what he sees.  This particularly works if one has a suggestion for what Jaynes might be missing; and I do.  I have in mind broadly memetics, and particularly the notion of verbal society which I suggested on this blog some time back and have revisited several times, notably [1], [2].

As for a work of consummate genius, well, that depends on one's view of genius.  If it's possible for a work to be a masterstroke regardless of how much of it is right or wrong, then, why not?  It's easy, when the Iliad says that someone did something because a god told them to, to say, oh, that's a poetic device; but in an academic climate where "poetic device" is the standard explanation, it takes something special to say — seriously, and with extensive scholarly research to back it up — that maybe, when the Iliad says a god told them to do something, the Iliad means just what it says.

Contents
Caveats
Background
The book that Jaynes wrote
Commentary
Storytelling
Caveats

When seeking to show an audience the plausibility of a paradigm scientific theory, it's common to point out things that are consistent with the theory.  However, if you're trying to show plausibility of a highly unorthodox scientific theory (the sort whose opponents might call "lunatic fringe"), imo that technique basically doesn't work.  My reasoning has to do with contrast between rival theories.

Imagine I've got a large whiteboard, with nothing written on it.  (When I was in high-school, it would have been a blackboard; and some years from now perhaps it'll be some sort of giant touchscreen technology.  At any rate, it's big; say at least a yard/meter high and wider than it is high, perhaps a lot wider.)  The points on this whiteboard are possible explanations for things; that is, explanations that we could, in principle, entertain.  I draw on it a small circle, perhaps the size of the palm of my hand.  The points inside the circle are tried and true sorts of scientific theories; we have repeatedly used experiments to test them against alternative explanations, and in that way they have earned good reputations as solid, viable explanations.  So if one of those well-reputed explanations works very well for some new thing you're looking at, it's a credible candidate to explain that thing.

What if none of the explanations in that small circle works for the new thing you're studying?  I'll draw a larger circle around that one, maybe four times the diameter.  Points inside this larger circle are explanations we have thought of, even if they seem quite loony.  There are flying saucers in there, and Sasquatches, and ancient aliens visiting Earth to build pyramids.  But they're all explanations that we've thought of, even if we didn't think highly of them.  Even the strangest among them, though, may have some people who favor them.  And when we've got a new thing that doesn't afford an explanation in the smaller circle, but it could be explained by, say, ancient pyramid-building aliens (to take a vivid example), some people will claim that's evidence for ancient pyramid-building aliens.

Except, it isn't evidence for ancient pyramid-building aliens.  It's consistent with ancient pyramid-building aliens, but ancient pyramid-building aliens don't have the earned reputation of things in the smaller circle.  Remember, those orthodox explanations earned their reputations through experiments that contrasted them against alternatives.  But when none of those orthodox explanations works for this new thing, and ancient pyramid-building aliens does work for the new thing, what alternative theories should we be considering?  Presumably, anything that has as much repute as ancient pyramid-building aliens.

Uhuh.

And this is why I've made these circles much smaller than the whole whiteboard.  The points in the larger circle are explanations we have thought of; but most of the whiteboard is outside that circle, and all that larger outside is explanations that we could consider, but we haven't thought of them.  And really, we don't know how much of that vast array of explanations we haven't thought of might be (if we thought of it) at least as well reputed as ancient pyramid-building aliens.

The moral of the story, it would seem, is that if you're studying a really unorthodox explanation, and you want to be able to say something stronger than just that it would suffice to explain the phenomenon, you should work at finding alternatives.

I don't mean to lambaste Jaynes for not coming up with alternatives; Jaynes was pulling off a profoundly impressive feat by coming up with one solid unorthodoxy, it's hardly fair to complain that he didn't come up with several.  But it does seem that however many facts he finds to be consistent with his unorthodoxy, one ought not to interpret that as support for the unorthodoxy, as such.  Throughout my reading of Jaynes, I kept this sort of skepticism in mind.

Another sort of trap for the unwary researcher in areas relating to the mind —orthodox or no— is highly abstract terms that really don't mean at all the same thing to everyone.  (The same sort of problem may arise in religion, another area with really extraordinarily abstract terms.)  I experienced this problem myself, some years ago, when reading Susan Blackmore's The Meme Machine.  Through most of the book I felt Blackmore seemed pretty much on-target, until I came to her chapter on the self; and when I hit that chapter it was quickly clear that something was going horribly wrong.  Suddenly, I found Blackmore saying things that on their face (the face presented to me, of course) were obviously, glaringly false.  And not just saying them, reveling in them.  She was quite excited, after having believed all her life that she had a self, to realize that the self did not exist.  This struck me as beyond silly.  If she was so sure she didn't have a self, who did she imagine had written her book?

I didn't take this to be, necessarily, a mistake by Blackmore; it didn't feel that way, though there wasn't any other explanation that felt compellingly right either.  But not chalking it up to a mistake by Blackmore did not in any way change the overt falsity of what she was saying.  Hence my initial phrasing, that something was going horribly wrong.

After considerable puzzling (about a week's worth), I worked out what was going wrong.  It wasn't a problem with the concepts, neither on Blackmore's part nor mine.  It was a problem with the word "self".  Susan Blackmore had believed all her life in... something... and was quite excited to realize that that something did not exist.  But she called that something "self".  And that thing, that she called "self", was something I had never believed in to begin with.  I had always used the word "self" to mean something else.  So when she said she had realized that the self does not exist, to me she was denying the existence of something quite different from what she intended to say did not exist.  I think she was denying the existence of what Daniel Dennett would call the audience of the Cartesian theater — which Dennett spent much of his classic book Consciousness Explained debunking.

The moral here would seem to be, don't assume that other people mean the same thing you do by these sorts of highly abstract words. 

Those two potential traps came to mind for me pretty quickly when I started reading Jaynes.  Another, more content-specific, trap occurred to me a few chapters into the book.  There is a well-known (in some circles) phenomenon that medical students, as they learn about various diseases, start worrying that they themselves may be suffering from those diseases.  I've inherited a story of someone remarking, about an instance of this phenomenon, "Just wait till they start studying psychiatry."  Well.  Jaynes was a psychologist.  There's this tendency to think in terms of pathologies.  And it seemed to me, as I got into the thick of the book, that Jaynes was placing undue weight on pathological states such as schizophrenia.  Without that emphasis, it seemed, one should be able to formulate a theory in the same general direction as Jaynes was exploring, without going to the extreme he went to (his bicameral man).

Background

Jaynes is concerned with the development of consciousness over time, and, peripheral to that, the development of language over time.

Some major milestones of human development, more-or-less agreed upon:

  • About two and a half million years ago, stone tools appear.  Start of the paleolithic (old stone age).
  • About forty or fifty thousand years ago, give or take, there is an explosion in the variety of artifacts.  Art, tools for making tools, tools with artistic flare, tools for making clothing, etc.  Start of the upper paleolithic (late stone age).
  • About ten thousand years ago (your millennium may vary), human agriculture begins.  Start of the neolithic (new stone age).
  • About four thousand years ago, the first writing appears.  This is a bit after the neolithic (perhaps a thousand years) and into the Bronze Age.
  • About 2500 years ago, around the time of Plato, science and philosophy blossom in ancient Greek civilization.  Eric Havelock proposed that this is when ancient Greek society passes from orality to literacy.
According to Havelock's theory, the shift from oral society, in which knowledge is founded on oral epics such as the Iliad, to literate society in which knowledge is founded on writing, profoundly changes the character of human thinking.  Modern Afghanistan has been suggested as an example of orality.

To Havelock's theory, I've proposed to add a still earlier phase of language and society, preceding orality, which I've tentatively called verbality.  My notion of what verbality might look like has been inspired by the Pirahã language lately studied by Daniel Everett as recounted in his 2008 book Don't Sleep, There Are Snakes: Life and Language in the Amazonian Jungle.  In particular, amongst many other peculiar features of the Pirahã, their culture has no art or storytelling, while their language has no tense and no numerical or temporal vocabulary.  It seems perfectly reasonable that the Pirahã would not be typical of verbality, since it's typical for a verbal culture to have vanished many thousands of years go; but I see it as a demonstration of possibility.  It may be significant for Jaynes's theory that Everett describes a Pirahã group hallucination.

I don't have a good handle on just what precipitated the ancient transition from verbality to orality — although, if one speculates that the story of the expulsion from Eden might be, in some part, a distant memory of the verbality/orality transition, it may have been pretty traumatic.  However, I do have a timeframe.  If verbality does not support art, one would expect the transition to be clearly marked by the appearance of art; besides which, I expect a dramatic acceleration of memetic development starting at the transition; so, I place the end of verbality and start of orality circa forty thousand years go, at the beginning of the upper paleolithic.

Once orality starts, about forty thousand years ago, it would then be necessary to work out increasingly effective ways to tell stories.  It seems likely to have been a very difficult and slow process; one would, on reflection, hardly expect ancient humans to immediately shift from not telling stories at all to great epics.  I'm guessing that writing, which didn't show up for about thirty six thousand years, was a natural development once the art of storytelling reached a certain level of maturity.  I really hadn't thought about oral society struggling to develop the art of storytelling, though, until I started reading Jaynes.

The book that Jaynes wrote

Jaynes begins with a chronological rundown of theories of consciousness.  This is good strategy, as it places his ideas solidly into context, allows the reader to see him doing so, and allows him to be seen considering alternatives, which helps not only the credibility of the theory, but also of Jaynes himself; not incidentally, as proponents of unorthodoxy need to be seen to be well-informed and attentive.  On the downside, his treatment of individual past theories tends to make light of them — although, I notice, on at least one occasion some chapters later, he acknowledges having just used such a tactic, suggestive that he perceives it as a perfectly valid stylistic mode and not something to be taken too much to heart.  I think he'd come across better by showing more respect for rival theories; at any rate, it's my preference.

His rundown of past theories seems likely to suffer from a problem, such as I described earlier, with the highly abstract term consciousness.  He's quite clear that these different theories are saying different things, but he appears to assume they are all trying to get at a single idea.  The difficulty might also be described in terms of Kuhnian paradigms (which I've discussed often on this blog, e.g. [3], [4]).  Amongst the functions of a paradigm, according to Kuhn, it determines what entities exist, what sorts of questions can be asked about them, and what sorts of answers can be given.  So, while the different paradigms Jaynes describes are all searching for truth in the same general neighborhood, one should expect that some of the variance between them is not merely about what answer to give to a single common question that all of them are pursuing, but about what question is most useful to ask.  As a reader, I struggled to deduce, from how Jaynes presented these past theories, just what question he wanted to answer; and I was still working on pinning that down after I'd finished the book.  His own notion of consciousness is, to my understanding, substantially about narratization, essentially telling a story about the self.  This notion of the self as a character in a story told by the mind seems fairly close to what I think of as self (as opposed to what Susan Blackmore apparently used to think of as self before studying memetics); and is clearly an application of storytelling (the thing that, by my hypothesis, is missing from verbality).

He is a good writer; reading his prose is — at least if you're interested in the subjects he's discussing — perhaps not a page-turner but nonetheless interesting rather than oppressive.

Following the introduction, he divides the work into three parts — Books I, II, and III — addressing the nature of the bicameral mind (Book I); the evidence he sees, burning across history, of the bicameral mind and its progressive breakdown (Book II); and the remnants of bicameralism he sees in our modern state (Book III).  He added a substantial Afterword in 1990, apparently when he stopped lecturing at Princeton, at the age of 70, and seven years before his death.

Jaynes's central idea is that for some time leading up to about 4000 years ago, human minds functioned along different lines than the narratization-based consciousness we experience today.  Instead, the human mind was, in Jaynes's terminology, bicameral.  The left brain (more properly the hemisphere opposite the dominant side of the body, but most people are right-handed) handled ordinary stuff, and when additional oversight was needed, the right brain provided a hallucination of someone telling the left brain what to do.  These hallucinations were perceived to be gods; or rather, in Jaynes's framework, by definition they were gods.  One illustration he mentions, from the Iliad, has an angry Achilles asking Agamemnon to account for his behavior, Agamemnon says a god told him to, and Achilles just accepts that.  The way Jaynes talks about these gods often makes them sound as if they were coherent beings, which struck me as an overestimation of how much coordination a civilization would likely be afforded simply by its population being bicameral.  Jaynes portrays a nation of bicameral humans as extraordinarily well-coordinated (in terms that sometimes seem to flirt with group selection, a particular pet peeve of Richard Dawkins that he spent most of his book The Selfish Gene debunking).

Jaynes's notion of bicamerality is extensively tied to his ideas about human language.  The area of the brain ordinarily responsible for language is on the left side of the brain but the corresponding right-side structure is largely unused; he figures that right-hand structure is where hallucinated voices came from.  His general view of the differing functions of the hemispheres is largely in line with, if distinctly more cautious than, the pop-psychology notion of analytic left brain and synthetic/artistic right brain (apparently the pop-psychology view had just gotten started a few years before Jaynes's book came out).  He has some specific notions about the stages by which human language developed, which I didn't fully absorb (too detailed, perhaps, to pick up while struggling with the big picture of the book on a first reading), though apparently he sees metaphor as key to the way full-blown human language works in general.  In a passage that stuck in my mind, he says that his linguist friends tell him human language is very old, stretching far back in the paleolithic (I've read estimates from a hundred thousand years all the way back to the start of the paleolithic at two and a half million years); he suggests this is implausible because things ought to have moved along much faster if language had been around during all that time, and instead he proposes language only started at the beginning of the upper stone age, forty thousand years ago.

He dates the start of bicamerality to the onset of agriculture, at the paleolithic/neolithic boundary, circa ten thousand years ago.  His reasoning (to the best of my understanding) is that to make agriculture work required coordination of large groups, and this coordination was achieved via bicameral gods.  For some thousands of years (nominally, about six thousand) this worked well, but then the world got more stressful, partly due to increasing population through agriculture enabled by bicamerality, and the gods couldn't keep up, forcing the development of the new regime of consciousness.

Commentary

Jaynes seems to me to be operating at a disadvantage.  Drawing inspiration from something he's familiar with, and viewing history through the lens of his individual perspective, he sees a pattern that he finds compellingly evident in history.  It seems — from my individual perspective — that a less extreme explanation for the historical evidence might well be formulated; but the less extreme explanation I see uses tools that weren't available yet when Jaynes was developing his theory.  Jaynes draws inspiration from his knowledge of the phenomenon of schizophrenics taking orders from hallucinations; which imho really is a delightfully bold move to shake up an orthodoxy that, like most orthodoxy, could do with a good shake-up.  But, Jaynes's book was published in the same year with Dawkins's The Selfish Gene, which coined the word meme.  It's easy to look back from four decades later and say that memetics can account for profound changes in population dynamics on a scale that Jaynes felt needed a radical hypothesis like bicamerality; but you can't stand on the shoulders of giants who haven't arrived yet.

Jaynes is concerned primarily, of course, with the breakdown of the bicameral mind, over time starting about four thousand years ago; he has little to say about the upper paleolithic, the thirty-thousand-or-so years from the start of language (by his reckoning, or the start of orality by mine) to the onset of bicamerality (by his reckoning, when the technological practice of agriculture was developed).  His later discussion of modern hallucinations describes them as vestiges of bicamerality, which rather begs the question of whether humans in the upper paleolithic had hallucinations.  The example of the Pirahã suggests to me that hallucinations —and language— were already part of the human condition even before the upper paleolithic.  (An interesting question for further consideration is whether the Pirahã's group hallucinations were non-linguistic.)

My own preference is for less radical transitions (consistent with Occam's razor).  Jaynes may be underestimating how much of qualitative consciousness can exist without narratization in the modern sense; how much of language can exist without support for art or storytelling; how far social structure may be determined by what is believed without involving any fundamental change in how belief is processed by the mind.  He also appears, in particular, to be underestimating how loosely organized the modern "conscious" mind is.  His view of consciousness is monolithic (something I particularly noted when he began to discuss schizophrenia in Book III).  Recall the atomic notion of self, which Susan Blackmore described rejecting after having previously believed in it.  If the self is a character in a story we tell ourselves, then the mind that's telling the story was never really atomic in the first place, and we needn't expect a mind that tells such a story to be fundamentally differently organized than one that doesn't tell such a story.  If hallucinations are somewhere within the penumbra of normal human mental functioning (and to my non-psychologist's eye it seems they may bear some kinship to narratization), it's possible for such phenomena to have had changing roles in society over the millennia without requiring a traumatic shift to/from a bicameral mind.

Another major pitfall he's at risk for concerns interpretation of evidence.  Our perception of the distant past is grounded in physical evidence, but we have to build up layers on layers of interpretation on it to produce a coherent picture, so that what we actually see in our coherent picture is almost all interpretation.  That leaves tremendous scope for self-fulfilling expectations in the sort of reasoning Jaynes is doing, where he reconsiders the evidence in light of his theory to see how well it fits.  Some of his remarks reveal he's aware of this, but still, there it is.  When he talks about how the meaning of a word changed over time, one should keep in mind that this is how he supposes it changed over time; the actual evidence is only written words themselves, while all the meanings involved are couched in a vast network of guesses.

The distinction between supportive evidence and consistent evidence is not absolute; it depends on how distinctive the evidence is — how much it calls for explanation.  This needs care when applied at scale.  Jaynes, in particular, examines a great pile of assorted evidence.  When the theory intersects with a sufficient mass of evidence, just being consistent with so much begins to seem impressive; but really one has to sum up over the whole mass, and the sum of very many data points can still be zero; it depends on the data points.

One doesn't want to give an unorthodox theory credit for explaining things that hadn't needed explaining.

Sometimes an explanation seems warranted.  Jaynes remarks of the Iliad that it never describes human bodies as a whole, but rather as collections of parts, and that the same trend is visible in visual art of the time; though that seems open to an explanation in terms of evolving technology for storytelling, it doesn't seem gratuitous to ask for some explanation of it.  Another point that gave me pause was his claim that the extraordinarily easy Spanish conquest of the Inca Empire was because the Inca Empire was bicameral, with the entire population following the dictates of their bicameral gods; though I didn't find it an altogether compelling case for his explanation, that chapter in history is odd enough that orthodox explanation isn't entirely at ease with it either.

Jaynes as a whole, though, gave me some general sense of unnecessary explanations.  He sees evidence of hallucinations where I see unremarkable phenomena (such as "Houses of God") that may be consistent with his theory but don't need it.  In Book III he is particularly keen on the idea that modern humans look for authority to take the place of the bicameral gods they have been deprived of; he sees a quest for bicameral authority in our attitude toward science (he's missing the difference between science and religion, btw, which may in part follow from predating memetics but I still found unsettling), and even sees the same quest for authority in our enjoyment of stage magic; but I have never felt that people looking for authorities to follow needed explanation.  I figure it's a basic behavioral impulse with some evolutionary value, rather like the impulse to be fair to others, or the impulse to hate people who don't belong to one's own social group (a very mixed bag, our basic behavioral impulses).  Yet more broadly, throughout the book he presents religion as a remnant of bicamerality.  Admittedly, this may come under the heading of things he missed by predating memetics; I now react to it by thinking, religion is neatly explained by evolution of memetic organisms — which ties in to the verbality/orality hypothesis — but I only made that evolutionary connection myself in the mid-1990s (earlier post).

Occasionally, in Jaynes's efforts to fit his theory to know facts, he encounters facts that don't fit easily.  Overall, this happens to him only sporadically.  He is aware that demonic possession doesn't fit his model, and tries to make it fit anyway.  He finds himself reaching to explain why poetry and music, which he maintains are remnants of bicameralism, still exist — which wouldn't be a problem if he hadn't started by hypothesizing they were remnants of a radically different type of mind rather than being phenomena within the normal range of the sort of mind we now have.

Storytelling

I look forward — after I fully digest my first reading of Jaynes — to a second reading.  My particular objective on a second reading would be to consider in detail how the evidence he claims for his bicamerality storyline fits with my verbality/orality storyline.  This objective wouldn't have been possible on the first reading, as I was too busy struggling to grok the overall shape of what he was saying; in fact, though I'd been accumulating thought fragments throughout his book, it wasn't until Jaynes's Afterword that I realized, in a definite Aha! moment (my notes pinpoint it at the top of page 458), that the key concept in relating Jaynes's theories with mine is storytelling, which underpins Jaynes's notion of consciousness and my notion of the verbality/orality transition.  So, as part of that full digestion, following is the more elaborated form that my theories have achieved from their first pass by Jaynes.

My narrative timeline, as it now stands (yes, theorizing is itself storytelling, which in this case feeds into the story being told since it implies that the advent of storytelling would produce a tremendous acceleration of human intellectual development), starts with the transition from verbality to orality at the beginning of the upper paleolithic.  Speculatively, this cultural transition may coincide in language development to the introduction of one or both of the two devices mentioned above as missing from Pirahã:  time, and numbers.  Jaynes's ideas about consciousness are rather close to those two factors, as well.  Once the orality-threshold device is introduced, whatever it is, there is a distinct expansion of human activity.

If the start of the upper paleolithic is when orality starts, it's a long time before the period Jaynes primarily discusses, as his breakdown of the bicameral mind starts only about four thousand years ago.  The intervening thirty six thousand years, be the same more or less, would have to be accounted for by the very slow process of inventing the art of advanced storytelling.  As mentioned above, Jaynes has little to say about this period.  He reckons language only began where I'm placing the verbality/orality transition, at the start of the upper paleolithic, and he (iirc) briefly describes a series of stages in the development of language that would have taken place during the upper paleolithic before the fully developed device of language catalyzed the emergence of the bicameral mind and the neolithic.  Some of Jaynes's language stages would likely precede storytelling, but certainly a second reading should carefully examine these stages in case some of them offer some inspiration on storytelling after all.  On the other hand, if he is indeed overestimating how much of consciousness must postdate his bicameral era, his timeline for the development of consciousness starting four thousand years ago might, on careful examination, be mapped more widely onto the entire oral period from (nominally) forty thousand to twenty five hundred years ago.

After the verbality/orality transition, the next specific event in my timeline is the emergence of writing, the point at which, by my conceptual framework, the art of storytelling exceeds a critical threshold enabling it to support the written form.  This coincides with Jaynes's start of the breakdown of the bicameral mind, four thousand years ago.  Jaynes's bicameral age is for me the late part of the larger oral period prior to emergent writing; his bicameral age might well be plausibly reinterpretable as a phase in the development of storytelling, perhaps something milder than but similar to bicamerality, though quite what that would be is unclear (and might stubbornly remain unresolved even after a second in-depth reading).

The period from the advent of writing onward is intensively covered in Jaynes's book, and wants close reconsideration from top to bottom.  Several complications apply.

Reinterpretations are likely to be steep in this period, with a wide conceptual gap.  In Jaynes's framework, bicamerality is an absolute state of mind with power to direct ancient empires, while religions are pale echoes of it; in mine, bicamerality is expected to fall within the normal operating range of the human mind (though perhaps not a part of the range commonly exercised in the modern era), while religions are memetic organisms with the power to direct ancient empires.

I remarked earlier on the treacherous nature of physical evidence with multiple layers of interpretation built on it.  A particular complication here is that Jaynes is judging what people think by how they describe their experiences, but I am hypothesizing that throughout the entire period people were trying to figure out how to describe their experiences, and in particular I'm guessing that explaining one's own thoughts was especially hard to figure out; so that the further back in time you go, the less people's descriptions reflect their inner life.

Judging by the above rough sketch of a timeline, the Iliad as we know it — even after compensating (or trying to) for mutation between being composed and being written down — should already represent an extremely advanced stage of storytelling, chronologically about seven eighths of the way from the onset of storytelling toward the present day.  Hopefully, a close second reading can use the depth of Jaynes's treatment to conjecture intermediate steps in the long evolution of advanced storytelling.