In a previous post, Darwinian Evolution, Chance and Design, I argued that the contention that Darwinian evolution occurs by chance does not entail that it shows the world was not designed. Once one sees how the concept of chance is defined in evolutionary theory one can see that it does not rule out design.
It is possible at this stage to argue that evolutionary theory rules out design by appeal to Ockham’s razor. Suppose one grants that chance is used in a limited sense to refer to the fact that, following Sober, “there is no physical mechanism (either inside organisms or outside of them) that detects which mutations would be beneficial and causes those mutations to occur.” Or following Mayr, it means, “that there is no correlation between the production of new genotypes and the adaptation needs of an organism in a given environment.” Suppose that evolution occurring by chance in this sense is well supported by the evidence. One could go on to interpret this theory in at least two ways; one might argue that evolution was in some sense guided by God or that it occurred by chance in a stronger sense that precludes design. Both arguments would be compatible with the empirical evidence. However, the sceptic could argue that the latter is preferable in virtue of the principle of Ockham’s razor, “entities should not be multiplied unnecessarily.”
The idea is that that the hypothesis of unguided evolution is simpler, more in accord with Ockham’s razor, than the hypothesis that God guided evolution. The former theory explains everything in terms of natural causes without appeal to a supernatural being such as God; hence, to claim God did it is to add an extra entity and is superfluous. Something like this argument is sometimes given by scientists writing in the field. George Gaylord Simpson writes,
Although many details remain to be worked out, it is already evident that all the objective phenomena of the history of life can be explained by purely naturalistic or, in a proper sense of the sometimes abused word, materialistic factors.
Similarly, Douglas Futuyma states,
By coupling undirected, purposeless variation to the blind, uncaring process of natural selection Darwin made theological or spiritual explanations of the life processes superfluous.
In this post I will respond to this appeal to Ockham’s razor and identify two problems with this line of argument. The first is that Ockham’s razor is only one criteria used to adjudicate rival hypotheses; the fact that one hypothesis is preferable on grounds of simplicity does not entail that it is preferable all things considered. There are at least couple of other considerations to take into account.
One other consideration to bear in mind is which hypothesis is more probable? To answer this question one would need to assess whether the probability that the history of life coming to be by a process of guided evolution is significantly higher than the probability of life coming to be by an evolution of blind chance and it seems to me, on the face of it at least, quite plausible to suggest that the chances of life evolving by chance is significantly lower than its coming about by divine guidance. If this is correct, then while atheistic evolution gives a simpler or more economical explanation of the origin of species it is considerably less probable.
A further consideration is that Ockham’s razor tells us that if two theories can explain the phenomena equally well then the one that postulates the least entities is the preferred one. For naturalistic evolution to be preferable to theistic evolution it would need to be argued that it explains all the data equally as well. The person who proposes the above line of argument, however, does not do this; all he or she does is show that both theories equally explain the history of life on earth. However, the history of life is only one feature of the world that scientists seek to explain. Showing that your theory explains one feature as well as a rival theory is a long way from showing that it explains all features as well.
This is important because one of the arguments theists offer is that God can explain numerous features of the world better, better than theism’s rivals. It has been argued that theism explains the origin of the universe, the continued existence of the contingent universe, the existence of laws of nature, the existence of cosmic fine-tuning, the existence and nature of objective moral duties and so on. To show that unguided evolution is a better theory then, one would need to show that unguided evolution provides a better explanation of all these phenomena together than theistic evolution does. If it does not then theism explains the history of life equally as well as atheistic naturalism but it also provides a better explanation of various other features of the world, thus it is a better overall theory.
So even if we grant that unguided evolution conforms to Ockham’s razor better than theistic evolution, it is not at all clear or obvious that this means it is a better theory, all things considered. Theistic evolution may be a significantly more probable theory and have significantly greater explanatory scope and comprehensiveness.
A second problem with the appeal to Ockham’s razor is that what constitutes a simpler hypothesis in a given context depends on the background information one has about what exists. Plantinga makes this point,
The theistic noetic structure already, of course, includes the existence of God. Relative to that noetic structure, therefore, there is no additional Ockhamistic cost in the hypothesis of guided evolution. As an analogy: suppose we land a space ship on a planet we know is inhabited by intelligent creatures. We find something that looks exactly like a stone arrowhead, complete with grooves and indentations made in the process of shaping and sharpening it. Two possibilities suggest themselves: one, that it acquired these characteristics by way of erosion, let’s say, and the other, that it was intentionally designed and fashioned by the inhabitants. Someone with a couple of courses in philosophy might suggest that the former hypothesis is to be preferred because it posits fewer entities than the latter. He’d be wrong, of course; since we already know that the planet contains intelligent creatures, there is no Ockhamistic cost involved in thinking those structures designed. The same would go for evolution; theists already accept divine design, and do not incur additional Ockhamistic cost by way of thinking of evolution as guided.
Plantinga’s insight is correct. He goes on to note that if we did not know the planet was inhabited by intelligent beings and the erosion hypothesis was not significantly less probable then an appeal to Ockham’s razor in favour of the erosion hypothesis would have “teeth.” If this argument works at all, it only works if one begins with an agnostic perspective and then postulates “the existence of a divine designer in order to explain the course of evolution.” If, however, one already knows that God exists and rationally believes God and has created the world on other grounds then we do not violate Ockham’s razor by appealing to the existence of God to explain the evolutionary process.
Evolutionary theory then might mean that an agnostic has reason for accepting atheistic evolution over theistic evolution as an explanation of the origin of species (I say “might” because the problem of relative probabilities and explanatory comprehensiveness still would need addressing). However, it does not show that those who believe in God, are irrational in doing so, nor does it provide a theist with any rational reason to reject his or her belief.
Again then, it seems that evolution by itself does not make belief in God untenable. Before it could the objector would first need to show there were no other grounds for believing in God other than the evolution of life on earth. Then he or she would need to show that one could construct a theory that has as much explanatory power as theism and on which the evolution of life by blind chance was not significantly less probable than its occurrence by divine guidance. Only after both these things were done could evolution be used to defeat theism. It is evident, however, that evolutionary theory by itself establishes none of these claims. Whether they are defensible is a substantive philosophical question over and above anything established in contemporary evolutionary theory.
This post draws from parts of my paper “Does Evolution Make Belief in God Untenable?” given at the recent TANSA conference, Faithful Science? – Just How Well Do Science and Faith Get Along?
 Eliot Sober “Evolution Without Metaphysics” (unpublished).
 Ernest Mayr Toward a New Philosophy of Biology; Observations of an Evolutionist (Cambridge: Harvard University Press, 1998) 98.
 George Gaylord Simpson The Meaning of Evolution (Yale University Press, rev ed., 1967) 344-345.
 Douglas Futuyma Evolutionary Biology (Sunderland, MA: Sinauer Associates Inc, 1986) 2.
 Alvin Plantinga “Science and Religion: Where Does the Conflict Really Lie?” (unpublished).