Realism and idealism are here treated in their philosophical senses, but nevertheless in relation to their practical implications. They are argued to be cognitive errors, mistakes we make in thinking alongside cognitive biases and fallacies, because they absolutise an uncertain experience into either ‘reality’ or its absence. Philosophical realism and idealism are related to the psychological processes of projection and idealisation, and to fallacies such as absolute generalisation and the straw man. This is the fourth of a series of ‘mistakes we make in thinking’, bringing together psychological and critical thinking accounts of our errors.