The plausibility of Naturalism - the view that the mind is a purely natural phenomenon that can be explained scientifically (if explained at all) - is a hotly contested topic in cognitive science, as every philosopher and cognitive scientist knows. It is not widely recognised, however that some of the more popular arguments against naturalism rest upon anti-realist metaphysical assumptions. This is a problem since the most plausible defences of naturalism presuppose a realist metaphysics. In this chapter, I shall chart one of these anti-realist assumptions and show how it features as a crucial premise in a leading anti-naturalist argument, the Knowledge Argument.