I'm working through R' Dovid Gottlieb's formulation of the Kuzari Argument, as laid out in his book LivingUp To The Truth and various articles. In his article TheKuzari Principle - Introduction, he says that no critic of his principle has ever provided the details of how the Sinai story could have developed if it hadn't actually happened. Instead, they wave vaguely towards myth formation. He even purports to do the critics' work for them, laying out how it might have happened, and then showing why it couldn't have happened that way. What he really does is set up a strawman where at some point someone tried to "sell" the story to the people, and counters this strawman with the argument that neither the first generation nor any of their descendants would have accepted a story of mass revelation if they hadn't experienced it themselves or heard about it from their parents and grandparents.
I think I have a plausible scenario for how the myth of matan Torah might have developed, with at least as much details as R' Gottleib provides in his strawman version. Best of all, it was inadvertently suggested by R' Gottleib himself.
In Living Up To The Truth, he sketches out his version of the Kuzari argument like this:
"Let's consider a possible event, that is to say an event about which we don’t know whether or not it occurred. Let’s suppose it is an event which if it had occurred, it would have left behind enormous, easily available evidence of its occurrence. Well, if we don't have the evidence then we will not believe it occurred.
That's what the principle says. Let's try to put it in simpler terms. Someone is trying to convince me that a war, or an earthquake, or something like that happened. If he is right that it (the war, earthquake, etc.) really happened, I should know about it already. I shouldn't need him to tell me. Then the principle tells me that I will not be convinced by him. The problem of the missing knowledge will prevent me from believing him.
Of course, when I say that "people will not believe," I don't mean that no one will believe. After all, there are people who believe in flying saucers, or that they are Napoleon, or that the Miami Dolphins will win the Super Bowl! What I mean is that you will not be able to get the vast majority of a nation to accept such a view about their own ancestors when no one in fact remembers it."
So a few people might believe, but you can't get a whole nation to believe something for which there isn't sufficient evidence.
A few pages later, he has a discussion about people who disbelieve in the Holocaust despite the overwhelming evidence, and says,
"…the Kuzari principle predicts that you cannot get people to believe that the Holocaust did not occur. But the prediction is in fact correct! More than ninety per cent of contemporary Americans believe in the Holocaust. The Kuzari principle does not say that no one will accept such a belief. For any kind of craziness you can find some believers! "
I think a plausible origin for the Sinai story lies with R' Gottlieb's crazy fringe, the ten percent or so who will accept something despite the lack of evidence that should be there. This origin works even in the scenario where story is "sold" to the people, as R' Gottlieb and other kiruv rabbis like to picture it, instead of organically developing from fables or exaggerations of real events in the way myths typically do.
In my scenario, the Sinai story starts with a charismatic religious leader who sells the story to the small group of "crazies" who will believe it despite the lack of evidence. Let's say, five percent of the population. They're eccentrics who no one takes seriously. But most of their children are not. The children grow up and become typical members of society, except that many of them retain the belief in the Sinai story they were taught as children. This isn't a belief that effects their lives in any significant way, so there's no reason to drop it. Everyone keeps the Jewish traditions, and they do to, with the insignificant difference that they believe the traditions were given to all the people all at once by God instead of revealed through prophets and leaders.
A few dozen generations later, everyone has forgotten that the Sinai story was sold to a small group of eccentrics. It's now a normal, albeit minority belief. The belief spreads through society as a result of friendships, marriages, etc., in the way that cultural phenomena typically do. In this way, the belief spreads to a few individuals at a time, who don't ask about the evidence for it. They are adopting it because it is the belief of the family they are marrying into, or because it is the belief of a significant number of their friends, and aren't concerned about intellectually justifying the belief. It's enough for them that this is what their family or friends believe. There is now a sizable minority that believes in the Sinai story.
Over time, the group or people within the group who hold the belief rise to prominence in society, and others adopt the belief in imitation of the social elites. Or the gradual spread of the story, one individual at a time, reaches a tipping point where there are enough people that believe it that it leads to most of the nation accepting it. They don't ask why there's no evidence for the Sinai story. They don't ask how everyone could have forgotten something so significant. Why would they, when clearly it wasn't forgotten. Even though a majority seem to have forgotten, there is a tradition, stretching back many generations, among a large minority of the nation that the ancestors of all the people received the Torah from God at Har Sinai. The expected evidence is right there for anyone to see!
A few millennia later, it's been forgotten that it was ever a minority belief, and R' Gottlieb insists that you can't ever convince people that a whole nation had an experience they didn't have.
R' Gottlieb is clever, and he leaves himself an out just in case a skeptic does come up with a plausible alternative to the Kuzari. He says that it's not enough to propose a plausible scenario. After all, there's lots of plausible things that didn't actually happen. To defeat the Kuzari, he says, the skeptic has to show that it happened. Otherwise, the Kuzari remains more likely.
Let's leave aside the near-impossibility of knowing exactly what happened in the distant past. I think R' Gottlieb defeats himself on two counts.
The first is the double standard he has regarding plausible scenarios. He demands that the critic show that the alternative to the Kuzari is what happened, or at the very least, that the critic show that the alternative scenario happened in the development of another culture's myth. But R' Gottlieb doesn't himself provide the kind of historical evidence he asks of the skeptic. Instead, he relies on the logic of his Kuzari argument and his contention that it couldn't have been accepted if it weren't true. And he insists that there is no other culture that has a comparable myth. So he's neither shown that the Kuzari is correct in regard to matan Torah or to any other culture's myths. It remains only a plausible scenario, and as he says, there are lots of plausible things that didn't actually happen.
The second is his appeals to our typical ways of knowing things. Throughout his formulation of his argument, he says things like,
" To violate the Kuzari principle we have to believe something for which all the expected evidence is missing. If it were true that there ought to be evidence, and there isn’t any evidence, we would never accept a belief. That is not part of our normal cognitive life."
Yet our "normal cognitive life," our regular way of knowing things in our everyday experiences, never looks to supernatural explanations. Even the miracles that people claim to experience are almost never supernatural experiences, but just really unlikely natural things, like the spontaneous remission of a terminal illness. If we are to appeal to our typical way of knowing, then any natural scenario, no matter how wild, is always going to be more plausible than a supernatural one. Any explanation I can think of, as long as it doesn't violate the laws of nature, is going to be more plausible than God having spoken to people. Even the strawman scenario of some charlatan selling the story to the whole nation. A whole nation accepting a story without any evidence that all of their ancestors heard God speak may be wildly unlikely, but it violates our "normal cognitive life" less than does a miraculous supernatural event where people audibly heard God speak to them.
R' Gottleib's formulation is far and away the most sophisticated version of the Kuzari Argument. And yet, it seems that it's self-refuting.
Kuzari book outline: Breaking the Kuzari