6. For this purpose, he must be skilled in three departments of science.
(a) In law—sufficiently to understand what are considered by the courts as characteristic marks of an insane mind, and what amount of sanity the courts require to hold a culprit responsible for his crime or a contract valid in its effects.
(b) In psychology—to such an extent that the expert witness can speak analytically and correctly as to the properties and actions of the human mind.
(c) In medicine—so far as concerns the treatment of the insane, and the understanding of their peculiarities, so as to reason from them by induction to the real condition of the client’s or patient’s mind.
But the main requisite for an expert witness is to understand clearly in what insanity properly consists, and how far it ought to excuse an insane man from bearing the consequences of his acts.
III. This two-fold knowledge is obtained by the psychological study of insanity, on which study we are now to enter, and it is the principal point in this whole matter.
Insanity means a want of soundness; he is insane whose mind is not sound, but is deranged, and therefore, like a machine out of order, it cannot properly perform its specific task, namely, to know the truth of things. An insane man cannot judge rightly.
1. Insanity takes various forms, which may be reduced to two kinds, with the doubtful addition of a third kind, namely, moral insanity, of which we shall speak in our next lecture.
The first kind consists in the total want or gross torpor of mental activity. When there is a total, or nearly total, eclipse of the intellect, the disease is called idiocy, the state of an idiot. When there is an abnormally low grade of the reasoning power, it is styled imbecility. The failure or decay of reason in old age is called dotage.
The second kind of insanity is called illusional or delusional. In it the intellect is not impotent; on the contrary, it is often unusually active; but its action is abnormal, its conclusions are false. Not that it reasons illogically or draws conclusions which are not contained in the premises. Very keen logicians may be demented. Their unsoundness arises from the fact that they reason from false premises; and they get their false premises from their diseased imaginations, whose vagaries they take for realities.