The Crucial Text
Most of the material below has been taken from Neal Stephenson’s novel Seven Eves. My strongest interest in this small slice of the novel is the expression with which it ends. It is: “consensual historical hallucination.”
I will add in this section the excerpts I have made. Then I will try to put the larger narrative frame in place. Finally, I will try to unpack this fascinating combination of words. Consensual historical hallucination? Really?
Tekla had been an easier target, where Aida was concerned, since she had stated so forthrightly what she considered desirable in a future race. It was easy enough to see that the children of Tekla were going to be strong, disciplined, formidable fighters. And one did not have to be a military genius to understand that fighting, for the foreseeable future—several millennia of being bottled up in space colonies—was going to be up close and personal.
Aida was not about to see her children dominated by the sons and daughters of Tekla. She might simply have done what Tekla did, and created versions of herself modified for certain traits associated with athleticism.
Instead, having become fascinated by the odd detail in her genetic report, she had embarked on a program to reawaken the Neanderthal DNA that, or so she imagined, had been slumbering in her and her ancestors’ nuclei for tens of thousands of years. It was a somewhat insane idea, and in any case she didn’t have enough Neanderthal in her to make it feasible, but she did produce a race of people with vaguely Neanderthal-like features, and in later centuries the processes of Caricaturization, Isolation, and Enhancement—which had affected all the races to some extent—had wrought especially pronounced changes on this subrace.
Gene sequences taken from the toe of an actual Neanderthal skeleton, found on Old Earth and sequenced before Zero, were put to use. Old Earth paleontology journals had been data-mined for stats on bone length and muscle attachment so that those could be hard-coded into the Neoander wet-ware.
The man sitting at the end of the table was the artificial product of breeding and of genetic engineering, but, had he been sent back in time to prehistoric Europe, he would have been indistinguishable, at least in his outward appearance, from genuine Neanderthals.
Their Neanderthal heritage had been fabricated out of whole cloth, yet it was taken more or less seriously by everyone—it was a sort of consensual historical hallucination.
To provide the context for this expression, I am going to have to fill in a very small part of the book. You need to know, so that you don’t get entirely lost, that all human beings (so far as we know for most of the book, including this part) have died except for these seven women,  who produce a “race” of people each.
You will see the names Tekla and Aida. They are two of the Eves and the races that derive from them are known as Teklans and Aidans. There are also subraces, one of which are called Neoanders, one of whom is referred to as “the man sitting at the end of the table.” His name is Langobard. This is Eve Tekla in the movie version.
So Langobard is an Aidan because he is of the genetic line of Eve Aida. He “is” a Neanderthal. Let me explain why I put “is” in quotes. He is the product of a great deal of genetic manipulation, but the starting point of all the manipulation is “gene sequences taken from the toe of an actual Neanderthal skeleton, found on Old Earth.” In addition, “Old Earth paleontology journals had been data-mined for stats on bone length and muscle attachment so that those could be hard-coded into the Neoander wet-ware.” Langobard is as close to a genuine Neanderthal as anyone will ever see.
Is he a “Neanderthal?” He is granted the status of Neanderthal. Without any question, the word “consensual” in the phrase we are unpacking shows that. “Historical” may refer to the period of history he is emulating or to the period where his status has been granted. I lean toward the latter.
But what is the hallucination? The principal referent of “hallucinate” is visual (the other senses may be involved, too) so it might refer to how Langobard looks. Except it doesn’t. You could plop him down on Old Earth in the company of naturally produced Neanderthals and he would blend right in. It is the “heritage” that is hallucinated.
Everyone has agreed, apparently, to grant that there is a body of people who are best thought of as modern day products of Old Earth’s Neanderthals, both of their physiology and their culture. Langobard’s actual “cultural history” comes from the time when there were enough Neoanders (modern Neanderthals) to build a culture, which could not happen before the genetic manipulations of Aida and the natural population distortions referred to above as “caricaturization, isolation, and enhancement.” Let’s say less that a thousand years of “history.” Not 5000, which separates Langobard from the actual DNA that was harvested to create his subrace. And Eve Aida, the progenitor of Langobard.
If this is what Stephenson has in mind, the “hallucination” is simply the exchange of the real Neanderthal history on Old Earth (surviving only in the DNA they harvested) for the real Neoander history of the most recent centuries. That is was “taken more or less seriously by everyone” is a choice they have made. As a choice, it cannot be a hallucination.
I think he has an agreed upon myth in mind. It is a “shared reality,” as in my title, but it is not a mistake. Nor is it true.
 Neal Stephenson, SevenEves.New York: HarperCollins, 2015, pp. 684-85 and 689-90.
 Ron Daniels is making the movie and you should see who he has cast as the Eves. I have included the pictures of the two whose names I use here.