We need to bracket abiogenesis away from biological evolution, which is where Behe is focused.
Yes, and that is exactly the point. Behe has not make a valid argument that apparently random evolution can’t increase the IC1 complexity of biological systems. In fact, we have shown in the lab, time and time again, that evolution can increase the IC1 complexity of systems.
This would only matter if arches with with 52 stones was a valid analogy of biological systems. It is not. So we do not need to worry about this in any precise detail. It is merely a loose analogy to help understand the deeper point.
That is, also, why I do not really care about the Stone Bridge example. You appear to be correct at pointing out its error in demonstrating the Muller Two Step. That was an ad hoc example that fails because it does not actually demonstrate a Muller Two Step. It is not hard to imagine, however, another version of the story that would, in fact, be a Muller Two Step.
One key falsification of IC1 is that the Muller Two Step is a clear mechanism for increasing IC1 complexity that Behe never considered when formulating IC1. Now that he knows about it, he never mentions this well known mechanism. With the the Muller Two Step in mind, it is no longer impaussible to imagine Biology evolving structures of increasing IC1 complexity. We also are now doubtful that IC1 complexity of the flagellum we se alive today has much at all to do with the simplest flagellum.
The other key error in reasoning is exaptation. There is no reason to think that all the parts must come together into flagellum to be useful. In the case of flagellum, there is strong evidence from several lines that different parts had different functions before they came together in a flagellum.
This means IC1 only appears like valid reasoning by:
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Ignoring clear examples of IC1 increasing in well understood experiments.
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Ignoring the Muller Two Step mechanism.
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Ignoring all possible prior functions for the constituent parts of an IC1 system.
Behe makes all these moves. To what extent he realizes he is doing this is another question entirely. Silence on these matters, however, is why people call IC1 an argument from ignorance. Just because Behe doesn’t know of a mechanism for evolving IC1 systems, does not some how evaporate the fact that biologists actually do know the mechanism by which IC1 increases.
Not that in Darwin Devolves there is NO MENTION of these difficulties in his argument. He just ignores them or dismisses them without reason.
In the public debate, as soon as it has become clear that IC1 fails, Behe switches to IC2, but without clarifying that IC1 failed. Of course, if you read him carefully, you will see that he does abandon it all the time, as merely useful to communicate with the masses, and not rigorous.