In his recent book Delusions and Beliefs, Kengo Miyazono offers a thoroughgoing defence of delusions as biologically malfunctioning beliefs, greatly elaborating on his earlier defence of this view. Miyazono has it that delusions have biological doxastic functions, and that delusions involve direct or indirect malfunctions of this kind. In this short piece, I focus on Miyazono’s defence of a two-factor approach to delusion formation as it appears in Chapter Four. Miyazono approaches his discussion of the debate between one- and two-factor (...) theories having already defended the key thesis of the book: that delusions are malfunctioning beliefs. Of course, that thesis might be thought to mesh nicely with the two-factor theorist’s claim that there is a cognitive abnormality present in delusion formation or maintenance. However, I will discuss Miyazono’s defence of the two-factor position in isolation from its role in his overall account of delusion. Miyazono abstracts away from the particulars of Max Coltheart’s two-factor view, and takes himself to be investigating the plausibility of two-factor theories without the specific commitments of Coltheart’s view. Miyazono also captures under the two-factor heading theories which locate the second factor in belief maintenance rather than just in belief formation. He puts forward a new argument for a two-factor approach which goes via inference to the best explanation. I begin by arguing that Miyazono’s starting motivation for a two-factor approach rests on a misrepresentation of the one-factor approach. Then, I turn to the four components of Miyazono’s inference to the best explanation argument, and argue that in each case, we do not have grounds for positing a second factor. (shrink)