The Evidential Argument from Biological Evolution: Part 1

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Many conservative Christians and lay atheists alike claim that if biological evolution is true, then God does not exist. Ironically, while many conservative Christians have attacked evolution because it supposedly entails atheism, only one contemporary atheist philosopher has argued that evolution is evidence for atheism: Paul Draper.

Draper defends an evidential argument from evolution for naturalism. In other words, Draper’s argument does not claim that evolution is logically inconsistent with the existence of God. Rather, it claims that known facts about evolution that are consistent with theism nevertheless provide evidence against it. Draper argues that, all other things held equal, known facts about the origin of complex life are prima facie evidence against theism.

This argument is focused on God in general, not necessarily the Christian God. This doesn’t make the argument irrelevant to Christian theism, however. Since Christian theism entails theism, the probability of Christian theism cannot be greater than the probability of theism simpliciter.

Informal Statement of the Argument

This argument assumes the truth of biological evolution; for a defense of that assumption, see the Talk.Origins archive. To be sure, biological evolution is logically compatible with theism; God could have used evolution to create life. But if theism were true, God could have also used many other methods to create life, methods which are impossible if naturalism is true. In contrast, if naturalism is true, evolution pretty much has to be true. Furthermore, since theism implies a metaphysical dualism, it is antecedently likely on theism that minds are fundamentally nonphysical entities and therefore that conscious life is fundamentally different from nonconscious life. But this in turn makes it likely that conscious life was created independently of nonconscious life–that evolution is false. Thus, the scientific fact of biological evolution is more likely on the assumption that naturalism is true than on the assumption that theism is true.

Formal Statement of the Argument

Definitions

genealogical thesis: complex life evolved from simple life

genetic thesis: all evolutionary change in populations of complex organisms either is or is the result of trans-generational genetic change.

evolution: the genealogical thesis conjoined with the genetic thesis

Darwinism: the much more specific claim that natural selection operating on random genetic mutation is the principal mechanism driving the evolutionary change that results in increased complexity

supernatural person: a person that is neither a part nor a product of the physical universe

perfect person: perfect in power (omnipotent), perfect in knowledge (omniscient), and perfect in moral goodness (morally perfect).

God: a perfect supernatural person

hypothesis: a proposition which we do not know with certainty to be true or false

theism: the hypothesis that God is the creator of the physical universe.

metaphysical naturalism: the hypothesis that the universe is a closed system, which means that nothing that is not part of the natural world affects it.

Note: Draper mentions Darwinism only to clarify that his argument is not an evidential argument for naturalism from Darwinism, which would be question-begging, but an evidential argument for naturalism from evolution (as defined above).

Draper’s Evidential Argument from Evolution

Logical Form

(1) Evolution is antecedently much more probable on the assumption that naturalism is true than on the assumption that theism is true.

(2) The statement that pain and pleasure systematically connected to reproductive success is antecedently much more probable on the assumption that evolutionary naturalism is true than on the assumption that evolutionary theism is true.

(3) Therefore, evolution conjoined with this statement about pain and pleasure is antecedently very much more probable on the assumption that naturalism is true than on the assumption that theism is true. [From 1 and 2]

(4) Naturalism is at least as plausible as theism.

(5) Therefore, other evidence held equal, naturalism is very much more probable than theism. [From 3 and 4]

(6) Naturalism entails that theism is false.

(7) Therefore, other evidence held equal, it is highly probable that theism is false. [From 5 and 6]

Legend

Pr(x): the epistemic probability of any proposition x

Pr(x/y): the epistemic probability of any proposition x conditional upon y

“>!”: “is much more probable than”

“>!!”: “is very much more probable than”

E: evolution

T: theism

N: metaphysical naturalism

P: pain and pleasure are systematically connected to reproductive success (cf. Draper’s argument from the biological role of pain and pleasure)

Draper’s Evidential Argument from Evolution Restated

(1) Pr(E/N) >! Pr(E/T).

(2) Pr(P/E&N) >! Pr(P/E&T).

(3) Pr(E&P/N) >!! Pr(E&P/T). (From 1 and 2)

(4) Other evidence held equal, Pr(N) >= Pr(T).

(5) Therefore, other evidence held equal, Pr(N/E&P) >!! Pr(T/E&P). (From 3 and 4)

(6) Naturalism entails that theism is false.

(7) Therefore, other evidence held equal, Pr(T/E&P) <!! 1/2. (From 5 and 6)

Draper’s Defense of His Premises

First Premise

Here, again, is Draper’s first premise:

(1) Pr(E/N) >! Pr(E/T).

Let S ≡ special creationism. E entails ~S. Therefore, E is logically equivalent to ~S & E.

Pr(E/N) >! Pr(E/T) iff Pr(~S&*E/N) >! Pr(~S&E/T)

Using axioms of the probability calculus, this becomes:

Pr(E/N) >! Pr(E/T) iff Pr(~S/N) x Pr(E/~S & N) >! Pr(~S/T) x Pr(E/~S&T)

Therefore, Draper’s strategy for showing that Pr(E/N) >! Pr(E/T) is to show that

A. Pr(~S/N) >! Pr(~S/T), and

B. Pr(E/~S&N) >= Pr(E/~S&T)

Draper’s Defense of A

N entails that S is false. So Pr(~S/N) = 1.

Given T, however, S might be true. So Pr(~S/T) <1. Therefore, Pr(~S/N) > Pr(~S/T). But, Draper observes, we can make a much more interesting claim than that, namely, that Pr(~S/T) <= 1/2. In other words, Pr(S/T) >= 1/2. The reasons for believing this are as follows:

  • “At first glance, it seems that the evidence for evolution is the only strong reason theists have for believing that God is not a special creator (which is to say that we don’t have any strong antecedent reasons for believing this).”
  • “We know by past experience that God, if He exists, has at least latent deistic tendencies.” Even independent of the evidence for evolution, the past success of naturalistic science does provide some reason for theists to believe that God is not a special creator.
  • Theists have a very strong antecedent reason for believing that God did create at least some complex life independently: the division between conscious and nonconscious life is enormously significant if theism is true.
  • “Before Darwin, many theists were special creationists.”

Thus, Pr(~S/T) <= 1/2. But notice that this entails that “~S is at least twice as probable antecedently on naturalism as it is on theism, which implies that it at least doubles the ratio of the probability of naturalism to the probability of theism.”

Draper’s Defense of B

The probabilities in B are to be assessed relative to the background knowledge that various complex life forms exist.

N entails that S is false, so ~S&N is logically equivalent to just N. Given that complex life exists, what makes evolution so likely given N is the lack of plausible naturalistic alternatives to evolution.

Given T, however, alternatives to evolution are somewhat more likely, simply because there is less reason to assume the complex must arise from the simple.

Therefore, Pr(E/~S&N) >= Pr(E/~S & T).

Second Premise

Recall that P represents the statement, “pain and pleasure are systematically connected to reproductive success.” Here, again, is Draper’s second premise:

(2) Pr(P/E&N) >! Pr(P/E&T).

In order to see why, let’s bring Darwinism (D) back into the conversation. Recall that D is the claim that natural selection operating on random genetic mutation is the principal mechanism driving the evolutionary change that results in increased complexity. D is much more probable given evolutionary naturalism than given theism.

Pr(D/E&N) !> Pr(D/E&T)

First, consider Pr(D/E&N).

  • D explains the increase in the complexity of life over time better than other potential naturalistic explanations;
  • D solves an explanatory problem for naturalism: the problem of explaining teleological order in organic systems;
  • Natural selection is just the sort of “blind” process one would expect to drive evolution if naturalism is true, since natural selection explains teleological order in organic systems without itself displaying such order

Second, consider Pr(D/E&T). Since evolutionary theism can explain teleological order in terms of God’s conscious purposes, it would not be surprising at all if the principal mechanisms driving evolution were such that they themselves displayed teleological order.

For these reasons, then, Pr(D/E&N) !> Pr(D/E&T).

Let us now return to the second premise:

(2) Pr(P/E&N) !> Pr(E&T).

Here are the supporting arguments.

  • E&N&D provide an antecedent reason for believing that pain and pleasure, like anything else produced by natural selection, will be systematically connected to reproductive success, which is what P states.
  • Our background knowledge includes the fact many other parts of organic systems are systematically connected to reproductive success.
  • Given E&T, however, P would be true only if the biological goal of reproductive success and some unknown justifying moral goal happened to coincide in such a way that each could be simultaneously satisfied. That’s a really big coincidence that E&N&D don’t need.
  • In fact, evolutionary naturalism (E&N) entails nothing that would provide an antecedent reason for doubting that pain and pleasure will resemble other parts of organic systems by being systematically connected to reproductive success. On the assumption that E&N is true, it would be extremely surprising if pain and pleasure appeared to be anything but morally random, whereas on the assumption that theism is true, a discernible moral pattern would be less surprising.

References

See Paul Draper, “Evolution and the Problem of Evil” in Philosophy of Religion: An Anthology (3rd ed., ed. Louis Pojman, Wadsworth, 1997), pp. 219-230; cf. Louis P. Pojman, Philosophy of Religion (Mayfield, 2001), chapter 6.

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