Let P be the process of genetically or mimetically producing belief in non-empirical claims in order to enhance social cooperation. Let moral realism be the claim that we know some non-trivial moral truths. Consider this argument:
1. (Premise) Evolutionary process P is the relevant process that produced both our religious and our moral beliefs.
2. (Premise) If all religious beliefs are false, P is unreliable (since roughly half of the basic types of beliefs produced by P are then false).
3. So, if all religious beliefs are false, then the relevant process that produced our moral beliefs is unreliable. (By 1 and 2)
4. (Premise) Beliefs produced by an unreliable relevant process are not knowledge. (This is a consequence of reliabilism.)
5. So, if all religious beliefs are false, we lack moral knowledge. (By 3 and 4 plus the analytic truth that knowledge requires belief.)
6. (Premise) If moral realism is true, we have moral knowledge.
7. So, if all religious beliefs are false, moral realism is false. (By 5 and 6)
8. So, if moral realism is true, some religious beliefs are true. (By 7)
And moral realism is true.
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