Delusion involves some anomalous experience, but the twofactor theory claims that a second factor is necessary to explain tenacity of delusion. In order to explore what the second factor is, I examine the processes of perception formation, belief formation, and belief evaluation. I conclude that perception formation and belief formation involve no abnormalities while belief evaluation involves an abnormality (the second factor) due to impairment of working memory. This is an abnormality of ignoring a hypothesis which mentions falsity of perception. In addition I suggest that abnormality in belief evaluation must not be severe to explain restrictedness of delusion while abnormality in experience (abnormal existential feelings) must be severe.
