The human personality is inextricably bound up with, among other things, the function of the central nervous system. Diseases and malfunctions of the brain, head injuries and neurosurgical operations can all result in permanently altered behaviour patterns. This interrelation between brain and behaviour is most clearly demonstrated in cases involving functional neurosurgery and severe traumatic lesions. Despite the fact that this interrelation represents an everyday challenge to the neurosurgeon, it is a question which receives less attention than it deserves in neurosurgical meetings. Given the scope and complexity of this topic, it is not possible to cover every aspect of it here: hence, discussion is limited to the impact on personality of injuries, language, epilepsy and psychosurgery. However, before considering the medical aspects, it was deemed necessary to try and arrive at a definition of "personality." This question was discussed by a number of philosophers representing various perspectives. Their diversity of viewpoints and conceptions greatly enriched the discussions.