To begin with, most assuredly he was never mad; at the most one could fairly say that, like most of the great leaders of thought, Luther was probably of the manic-depressive temperament, with that strange mixture of apparently insane egotism and gloomy pessimism that so marks people of that temperament. In his famous prayer he orders his God about in a way that one can only compare to that of the Presbyterian divine who cried in a moment of irritation, “Noo, Lord, that’s fair ridic’lous.” If you read Luther’s _Table-talk_, you will at once be struck with his curious temperament, which could combine a certain amount of shrewd common sense, such as you would expect from a man of Saxon peasant stock, with profound belief in the supernatural, a good deal of disbelief in his fellow-man, virulent hatred of the Pope and all his works, and a good deal of what looks uncommonly like sheer mysticism. _En passant_ I found therein the solution of a problem that has long puzzled me. What was the mysterious “sin against the Holy Ghost” that nobody seems to understand? Let Luther explain it to us himself. Many persons have imagined that it represented one of those sexual perversions against which primitive races have so often launched a fierce tabu, simply because they knew nothing of sexual pathology. But really, according to Luther, it was nothing of the kind. “Sins against the Holy Ghost are: first, presumption; second, despair; third, opposition to and condemnation of the known truth; fourth, not to wish well but to grudge one’s brother and neighbour the grace of God; fifth, to be hardened; sixth, to be impenitent.” The only fault one has to find with this is that Luther does not tell us how to recognise the truth when one sees it. What _is_ the criterion of truth? Otherwise it would seem to be a fairly good description of a certain type of neurasthenia. Many neurasthenics must go in mortal sin every day of their lives, for it is well known that the devil is particularly on the lookout for sins against the Holy Ghost. Probably Luther’s devil merely represented symptoms due to his wretched health. There is an excellent description of his dystrophy in Hartmann and Grisar’s monumental _Life of Luther_, and Dr. Cabanes went over it again from the point of view of modern medicine; while nearly fifty years ago Dr. W. W. Ireland of Edinburgh reviewed it from the point of view of an alienist of that time. But Ireland did not perceive the immense influence of Luther’s physical ailments on his mental condition. How could you expect him to, fifty years ago? From these three sources, therefore, I draw the material for this essay. A précis of Dr. Cabanes’ essay appeared in the _St. Louis Urologic and Cutaneous Review_ for November, 1924.