 Increasingly, explanations of delusions are multi-factorial. Influential early models that grew out of research on single-themed delusions resulting from brain damage, such as Capgras, hypothesized two deficiencies: an initiating aberrant experience or perception due to brain dysfunction, combined with some failure of the system of “belief fixation” by which initial beliefs are critically reviewed in light of other information (Davies et al. 2001; Davies &amp; Davies 2009; Young 2011). The second factor in such “two factor” theories is faulty inference, and is on some accounts distinguished by degree alone from common forms of reasoning error such as attribution bias (Bentall 2004). On an alternative, “prediction error” model, perceptual aberrations conflict with prior (Bayesian) expectations when accompanied by reasoning bias, giving rise to delusions (Stone &amp; Young 2007; Mishara &amp; Corlett 2009). The “polythematic” and complex delusions more often seen in the psychiatry clinic, that are partially driven by social and environmental effects, present significant challenges for cognitive science, since they involve the higher cognitive systems that apparently resist modular analysis and decomposition.