 For two-factor theorists (Davies et al. 2001; Stone and Young 1997), the delusion is formed in order to explain a puzzling experience or a failed prediction, but the presence of the experience or the failed prediction is not sufficient for the formation of the delusion. The mechanism responsible for the formulation of the delusional hypothesis must be affected by reasoning biases or reasoning deficits. Developments of this theory have been offered by Aimola Davies and Davies 2009, by Coltheart et al. 2010, by McKay 2012, and by Davies and Egan 2013.