ID proponents are often accused of using “God-of-the-gaps†arguments. Of course, there are positive arguments for inferences to design in the natural world, but Del Ratzsch makes an interesting point about gap arguments in this interview.
He comments:
…the SETI program is a gap-searching project — trying to find signals which nature alone couldn’t or wouldn’t produce, then constructing alien-civilizations-of-the-gap arguments. Further, it is nowhere written in stone that nature has no causal or explanatory gaps of the relevant sort… gaps and gap arguments as such are unproblematic in principle.
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…gaps have to do with e.g. mechanical causal histories, whereas design has to do with intentional histories. Those are in many cases intimately related issues. Gaps can be important clues to design, since depending on the context an actual mechanical, causal gap could suggest agency as a causal factor, and it is a relative short step from there to design. But the issues are distinct, and the ritual allegation that design views are all God-of-the-gap theories is inaccurate philosophically, as well as historically and contemporarily.
…It is also worth noting that if nature is designed and if it does contain causal or explanatory gaps, then any prohibition on gap theories will nearly guarantee that science — discarding one failed non-gap theory only by replacing it with another (not yet failed) [non]-gap theory — will not self-correct in the usual advertised way, and that science will never correctly understand the relevant phenomena.
Ratzsch points out that the search for extraterrestrial intelligence is based on a gaps-argument premise: If it could be shown that a signal from outer space could not possibly — or at least could not rationally or reasonably — be the product of natural law or chance, it would be legitimate to conclude that it was designed by an alien intelligence.
One might then ask, If it could be shown that life could not possibly — or at least could not rationally or reasonably — have spontaneously generated from non-living matter through natural law or chance, wouldn’t it, by the same logic, be legitimate to conclude that it was designed by an intelligence?
Note Ratzsch’s point in the last quoted paragraph. If, in the SETI case, it turned out that a long series of prime numbers were detected in a signal from outer space, and the gap-argument logic were disallowed, one might enter into a long series of just-so naturalistic stories to explain away the prime numbers and never arrive at the truth, which is that ETs actually do exist.
Ratzsch also points out in the interview that, although materialistic science has filled many gaps in our understanding, there is no guarantee that it can do so in every case. He notes that the gap presented by cosmological fine-tuning gets increasingly bigger the more we learn.
As a final comment, note that in mathematics there is an analog of gap-argument logic, and it’s called proof by contradiction. Since we’re on the subject of prime numbers, here’s an example of how it works:
I might not be able to directly prove that there are infinitely many prime numbers, but what if I could prove that there could not possibly be a largest prime number? Let’s assume that there is a finite number of primes and that P is the biggest one. Multiply all the prime numbers less than or equal to P together, and then add 1. We’ll call this number N. N is not divisible by any prime less than or equal to P, because when you divide N by any of those primes you’ll always get a remainder of 1. N is therefore either prime, or it is divisible by a prime bigger than P. In either case, P is not the biggest prime, and there cannot possibly be such a thing.
We’ve just found a gap that can only be filled by an infinitude of prime numbers.