Abstract
In choosing the subject for this Lecture commemorating the august personality of Henry Maudsley, I did not intend to be exhaustive and I had in mind two things. I would like to discuss how the study of intellectual defects and their objective measurement may contribute to the knowledge of what Maudsley (1895) called the pathology of mind. Also, I would like to draw attention to the way in which clinical work in mental deficiency may help to explain some of the causes of mental illnesses. The first topic is psychological, the second is biological.