In the 18th century it was assumed that the mind was 'transparent to itself '- that everything occurring in the brain was available to conscious thought. This idea seemed much less plausible after Freud introduced the notion of an unconscious mind housing 'dangerous' thoughts and desires. Modern psychology, though, has introduced another picture of unconscious mental processes, which has nothing to do with conflict, guilt or repression, but without which it is almost impossible to account for some of our distinctively human abilities, such as language. Professor Jerome Fodor, from the Massachusetts Institute of Technology, discusses how this other non-Freudian unconscious might be organised, and how it relates to our conscious thinking.