The attribute of the insane patient which is at once the most general, the most obvious, and the most striking, is his apparent irrationality. It is so evident that the delusions he exhibits are false beliefs, that the hallucinations have no objective reality, that the depression or exaltation is totally unjustified by the actual state of affairs. He is, moreover, so plainly impervious to the contradictions which his experience everywhere presents to him, and so absurdly obtuse to every argument and demonstration which our wits can devise. In the face of all this the superficial observer can only conclude that the root of the evil lies in the patient's incapacity to see reason, and that these grotesque symptoms have arisen simply because the mind has lost its ability to think rationally. To such an observer the essence of the matter is that the reasoning powers are diseased, and hence that the mind is capable of thinking any thought, however absurd and baseless it may be.

A more careful investigation, however, soon reveals facts which it is difficult to reconcile with this view. In many patients the reasoning powers seem to be in excellent order so long as they are applied to matters not immediately connected with the delusional system. Thus a patient who is firmly convinced that he is the son of George III may be capable of solving the most abstruse mathematical problems, and may perhaps fulfil duties in the hospital which demand the utmost nicety of judgment and discrimination.