by humans, we would not infer that the watch found on the heath is designed. But surely,
even if one found some mysterious complex interrelated mechanisms, ones with the
complexity of a watch, on a different planet, where one knew that they were not designed
by humans, one would infer the existence of an intelligence behind them. Thus, the
inference of design does not depend on its being human designers that are inferred.
Rather, the inductive data of seeing humans construct artifacts open our eyes to seeing
how intelligence in general functions and what products rational agency produces. And,
in any case, Hume's reply fails if the form of the argument from design is not analogical
but, say, that of an inference to the best explanation.
However, the most powerful blow against Paley's argument was not struck by Hume but
by Charles Darwin, who argued that the mechanisms that impressed Paley so much
probably were generated by the natural process of organisms mutating and only the fitter
ones surviving to reproduce. Nondeductive teleological arguments can be challenged in
various ways. One of the ways is to show the existence of a satisfactory explanation of
the items in question by a nondesigned natural process, since that would challenge the
claim that the theistic explanation is the only or the best one available. It might well be
that both a theistic and a naturalistic explanation are true, but in the presence of a
naturalistic one, the theistic one may not be needed or may not be the best one. Of course,
for the naturalistic explanation to be satisfactory, the naturalistic process cannot be an
improbable one. It will not do to explain the existence of a watch by saying that the
molecules making it up randomly came together under the influence of quantum
randomness, because this process would be ridiculously improbable. How
end p.129
ever, the Darwinian claim is that mutation plus natural selection makes the existence of
complex biological mechanisms probable.
The Darwinian account does not deal a deathblow to Paley-type arguments. First of all,
evolution does nothing to explain why there were living organisms on earth in the first
place. Evolution only functions when a self-reproducing entity is on the scene: it cannot
explain the coming-to-be of such entities. And prima facie we would expect that any self-
reproducing organism would have a certain minimal complexity. The simplest
independent living organism we know of is the Mycoplasma genitalium, whose genetic
code comprises 517 genes, with the DNA consisting of about 193,000 codons, each of
which can code for one of twenty amino acids. Experiments suggest that only about 265
to 350 of the genes are needed for life (Hutcheson et al. 1996). But even the 265 shortest
genes would have a total length of 4,239 codons.^2 Because each codon codes for one of
twenty amino acids, this gives us 20^4239 ≈ 105515 possible DNA sequences of this length,^3
and the chance that a random DNA sequence of the appropriate length would be
equivalent to the particular sequence of one Mycoplasma genitalium organism is thus less
than one in 10^5515.
We can call an event whose probability is less than 10^100 “astronomically improbable,”
since it would not be likely to have been generated in the 12 to 18 billion years our
universe has been around, even if each of the molecules in the universe, there being no
more than about 10^80 of them, tried to randomly produce the event a hundred times a
second. In practice, other DNA sequences could produce an organism with the same