Freud's book On Aphasia holds a special place in his contributions to neurology. It was his first writing on mental activity, emphasizing the functional aspects and paving the way for Goldstein's most fruitful concept of the problem. The concept of the 'speech apparatus' was one of a series of formulations lying on the borderline between the organic and the psychic, therefore of special importance at that time. Various terms in the book later became useful in analytic terminology, and the ideas of overdetermination and regression were here defined for the first time. He expressed a deep debt to Hughlings Jackson, who had written brilliantly on the inferences one might draw from the errors and stereotyped phrases of aphasics, and on other aspects of the problem. There is ample evidence that some of Jackson's concepts contributed to the basic theory of psychoanalysis.