The major advances in knowledge are made by experimental investigation, but it is sometimes forgotten that this must be preceded by experiment in thought, and it was in this that Jackson's genius was transcendent. From clinical observation he arrived at questions that have provided the material for an immense amount of experimental research in the laboratory, as well as guiding clinical enquiry in profitable directions. To take examples, his conclusion that positive symptoms from destructive lesions must arise from the uncontrolled activity of the healthy parts remaining–simple as it now appears–was the starting point of a revolution in thought which has ever since inspired neurophysiology. Again, if we consider our present knowledge of the epilepsies, Jackson's concept of the discharging lesion and his idea that there are as many different kinds of epilepsy as there are sites of discharge were the seeds from which the trees have gr