The Argument from the Biological Role of Pain and Pleasure

Informal Statement of the Argument

The naturalistic explanation for this is obvious. If animals are the products of evolution by natural selection, we would expect physical pain to aid survival. But not all physical pain and pleasure aids survival. For example, think of the horrible pain that inflicts many people with terminal illnesses. If naturalism is true, this is what we would expect: evolution by natural selection is not an intelligent process; there seems to be no way for creatures to have evolved so that they only feel pain when it will aid survival. In contrast, if theism were true, God could "fine tune" humans so that they experience pain only when it is necessary for some greater good. If God did exist, what possible reason could he have for allowing people with terminal illnesses have to endure such agonizing pain until they finally die? The chances that such a reason would intersect with the biological goal of survival is pretty slim. Thus, the biological role of pain and pleasure is more likely on naturalism than on theism.


Formal Statement of the Argument 


Here is a formal statement of the "argument from the biological role of pain and pleasure," as formulated by agnostic philosopher Paul Draper.

Let ">!" to mean "is much greater than"

HT = hypothesis of theism

HI = hypothesis of indifference: neither the nature nor the condition of sentient beings on earth results from benevolent or malevolent actions performed by supernatural persons.

-- Note that HI is consistent with both metaphysical naturalism and supernaturalism

-- Note that HI and HT are mutually exclusive

Let O = observations of humans and animals experiencing pain and pleasure. O can be broken down into three specific observations:

O1 = moral agents experiencing pain or pleasure we know to be biologically useful

O2 = sentient beings that are not moral agents experiencing pain or pleasure that we know to be biologically useful

O3 = sentient beings experiencing pain or pleasure that we do not know to be biologically useful

O is equivalent to O1 &  O2 & O3.

Let h refer to any generic hypothesis.

Pr(O/h) = Pr(O1 & O2 & O3/h)= Pr(O1/h) x Pr(O2/O1 & h) x Pr(O3/O1 & O2 & h)

So, it follows that:

Pr(O/HI) = Pr(O1/HI) x Pr(O2/O1 & HI) x Pr(O3/O1 & O2 & HI)
Pr(O/HT) = Pr(O1/HT) x Pr(O2/O1 & HT) x Pr(O3/O1 & O2 & HT)

In order to show that Pr(O/HI) >! Pr(O/HT), we can show that each of the terms on the right hand side of equation A is greater or even much greater than each of the corresponding terms on the right hand side of equation B.

(1) Pr(O1/HI) >! Pr(O1/HT)
(2) Pr(O2/HI & O1) > Pr(O2/HT & O1)
(3) Pr(O3/HI & O1 & O2) >! Pr(O3/HT & O1 & O2)
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(4) Therefore, Pr(O/HI) >! Pr(O/HT). (from 1, 2, and 3)
(5) O is known to be true.
(6) HT is not much more probably intrinsically than HI.
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(7) Therefore, other evidence held equal, HT is probably false.

Note that while the above argument implies that we have a good prima facie reason to believe that HT is probably false (since HT and HI are incompatible), it does not imply that we have a good prima facie reason to believe that HI is true (since HT and HI are not jointly exhaustive and so could both be improbable). So the argument from the biological role of pain and pleasure could be more accurately described as an argument against HT than as an argument for HI, though of course in some sense it is both.