From a Darwinian point of view, human beliefs are adaptations to our part of the world. No doubt much of what we believe must be roughly accurate, or else we would not have survived. But the beliefs we have evolved might latch on to the world only enough to help us stumble our way through it, and then only for the time being. Human belief-systems could be useful illusions, appearing and disappearing as they prove to be more or less advantageous in the random walk of natural selection. Might not evolution be one of these illusions? Scientific naturalism is the theory that human beliefs are evolutionary adaptations whose survival has nothing to do with their truth. But in that case scientific naturalism is self-defeating, since on its own premises scientific theories cannot be known to be true.