Kant’s objections to Cartesian accounts of the mind in the first Critique often lead readers to assume that he endorses some form of materialism, but the discussion of psychological ideas serves, Kant claims, to “destroy completely all materialistic explanations of the inner appearance of our soul” (4:334). As the chapter shows, the Prolegomena makes a distinctive appeal to the regulative use of these psychological ideas to argue against psychological materialism, or the view that our psychological states can be explained in terms of materialistic grounds, and further buttresses – in a different fashion – the position that Kant develops in the ‘Paralogisms’ section of the first Critique.