Ames also developed the concept of "transactional ambiguity" holding that "mental set" or expectation could materially affect one's perception of visual and other stimuli, as with the Ames trapezoid. This hypothesis extended the impact of mental set from the widely believed impact on one's conclusions about stimuli (the eye witness phenomenon) to actual perception of the stimuli itself. If true, it calls seriously into question the value of eye witness reports even by individuals with no prejudices about their observations. In 1941 Ames began to make notes in the morning on his transactional analysis of perception. In 1960 his collaborator Hadley Cantril published an edited selection from these notes, with a Preface, and included Ames' correspondence with John Dewey.