152 J.J.C. Smart
this danger. To argue for theism on the basis of gaps in scientific explanation
is a risky endeavour, since the gaps may be filled in. Thus Newton held that
God would have to readjust the motions of the planets from time to time as
the perturbations due to their mutual accelerations built up. Later La Place
proved the stability of the solar system.^4 E.W. Barnes was a fine mathematician
who became a theologically modernist and sceptical bishop. Nevertheless,
more than sixty years ago he wrote ‘The mystery of life is unsolved, probably
insoluble’.^5 If he had known of all the developments in biochemistry and
molecular biology that have occurred in more recent times he would no doubt
have thought the mystery to have at least been greatly diminished. However,
Haldane holds that he has philosophical arguments for certain of the gaps, and
that since the arguments are a priori or apodeictic they will not be overturned
by developments in biology or other sciences.
I find Haldane’s philosophical argument against the emergence of the
reproductive from the non-reproductive unpersuasive (see pp. 93 – 6). Why
could not a self-replicating molecule come about through the coming together
of a number of non-replicating molecules? No doubt this would have been a
very rare event but the universe is immensely large and was in existence for
a long time before the beginning of life. Of course such small proto-replicators
would have to evolve by natural selection into the DNA molecules of present-
day life. But I see no impossibility in this. Haldane thinks that self-replicating
molecules need pre-existing channels of information (see pp. 92–3) and this
produces a circularity or unacceptable regress in the physicalist account. As
far as I can tell, there is no talk of channels of information in contemporary
accounts of self-replicating molecules. They just replicate. Of course they do
require a sea of common molecules from which to build up the replicated
molecules.
This illustrates an important methodological point. When confronted with
some alleged gap in the story of the evolution of life, I do not feel constrained
to point to some well tested theory of how the gap was filled. It is enough for
me as a naturalistic philosopher if I can point to reasonable speculations as to
how it might have been filled. These speculations will have to be informed
by well tested theory but they would be speculations none the less. There
might be more than one speculation about the origin of life. (For example,
the recent discovery of various sorts of organic molecules in interstellar space
might or might not be relevant.) If there is only one plausible speculation we
are to some extent warranted in believing that this is in fact how things
happened. As a philosopher I am happy enough if we can see that the origin of
life is not impossible according to physical principles and cosmological know-
ledge. We do not need a detailed theory of it to prefer a naturalistic explana-
tion (thin and as yet speculative as it may be) to a supernaturalist explanation.
It would be nicer for me, as a naturalistic philosopher, to be able to point to