The authors describe Bion's use of the notion of the 'selected fact' in the evolution of analytic interpretations. They draw attention to the similarity between the emergence of a 'configuration' from a selected fact and the crystallisation of delusional certainty from an 'overvalued idea'. The risks to the patient of the imposition of an overvalued idea is described. They emphasise the importance of monitoring the subsequent development in sessions following interpretations for evidence of conscious and unconscious reactions to it and offer clinical examples of the use of a selected fact in arriving at an interpretation and the unconscious use of an overvalued idea to form an interpretation and the subsequent analysis of its effects.