What leads to the Argument From Personal Incredulity?

I just finished reading the slim account of a debate between Dan Dennett and Alvin Plantinga, entitled “Science and Religion: Are They Compatible?”.

Dennett’s conclusion is straightforward:

Plantinga wanted to show… that science and religion are not just compatible: Science depends on theism to underwrite its epistemic self-confidence. […] [But] Our capacity to discover the facts, and to have good reasons for believing that we have done so, is explicable without appeal to inexplicable or irreducible genius, immaterial minds, or a divine helping hand. […] Let Plantinga, like Behe, try to show us the irreducible complexity in our minds that could not possibly have evolved (by genetic and cultural evolution). He will find, as Behe has, that his inability to imagine how this is possible is not the same as a proof that it is impossible. Richard Dawkins calls this the Argument from Personal Incredulity, and it is an obvious fallacy.

This fallacy — the Argument from Personal Incredulity – is one that has always fascinated me. Of course many (?most) people who say that they “can’t imagine how X could have happened” have probably never actually tried to imagine it: they have a prior commitment to a position that holds that it could not have done so, and that’s good enough for them. And in some cases they may lack the background to actually reason about the subject. But how about the others? What factors might underlie an honest, good-faith Argument from Personal Incredulity concerning the reality of evolution, from the origins of life to human consciousness?
It seems to me that there are a number of failures of imagination which, while individually innocuous, could have a cumulative effect. Let me list a few of them; I’m sure that you can think of others. Each of these items warrants a lengthy exposition, but for now let me simply summarize:

  • Lack of appreciation of very big (and very small) numbers. The enormous age of the planet (143 Petaseconds). The vast number of cells in an organism – or on the planet. The short time needed for a chemical reaction to play out, or for a mutated gene to thrive or be extinguished.
  • A tendency to think backwards, deterministically, all-or-nothing rather than forwards, incrementally, in parallel, with contingency. This builds on the large numbers involved: the entire planet (and indeed the universe) is full of experiments in selection, all proceeding in parallel, all interacting, and all subject to myriad contingencies.
  • Lack of appreciation of how much can be accomplished by so little. To understand the power of complex systems, start with simple ones. Look at the range of capabilities of single-celled organisms, or artificial life systems. Now apply scale and parallelism to these simple components.
  • Hubristic over-confidence in the capacity and ineffability of the human mind. Our brains are good at handling fragments of analysis, recognition, inference, and recollection; they’re also quite good at weaving together a story to fill in the gaps and fix up the mistakes. We “remember” things we haven’t seen, and “decide” to do things that have already started. (Philosophical arguments about mental capability – things like Searle’s “Chinese Room” and Jackson’s “Mary” – assume a total competence which is demonstrably at odds with the way the brain actually works.)