The free agency of the individual is affected or modified in many different ways by the different diseases of the mind, and the question of responsibility will often be found to be one of the most perplexing problems with which the physician has to deal. If well-marked forms of insanity alone were to be investigated, the matter would be comparatively a simple one; but such is far from always being the case. The insane man often commits certain crimes precisely as an ordinary sane criminal would do the same thing. Often the evidence is contradictory, the testimony as to previous life and character conflicting, and the disease of so obscure a stage or type that it is almost impossible to form a clear opinion. The determination of a man's degree of free agency is no simple affair which can be decided in all cases by a few or a few dozen interviews. Not seldom the mystery remains unsolved after the autopsy. Man's free will is not the property of any substance which can be demonstrated by chemistry, physiology, or microscopical research, but it is the result of the combined action of a whole group of functional activities the very relations of which to each other are as unknown as their method of action. No stethoscope or ophthalmoscope can reveal its morbid action, which can only be inferred indirectly from the operations of the mind. The cases in which the physician is called upon to define insanity as the term is used by the lawyers are (1) to secure limitation or control of an individual's actions, usually by a guardianship; (2) to control him absolutely in an asylum; (3) to estimate his culpability or criminality, or {102} his capacity to make a will or contract or to transact business. It is quite important, therefore, that the medical man should understand that there may be, as regards some particular person, a wide difference between medical insanity or mental disease and legal insanity or irresponsibility. He does most wisely when he confines his testimony to an explanation of the changes caused by disease in the particular case, and to the effect of such changes upon the mind, leaving to the judge's charge and the jury's verdict the questions of guilt and responsibility.