Respecting the quantum of reason which the lunatic possesses, the physician may safely conclude, where he betrays no derangement, that on such topics he is of sound mind; and the fact is firmly established, that those who are insane on particular subjects, will reason correctly on ordinary and trivial points; _provided they do not become associated with the prevailing notions which constitute their insanity_. Ordinary persons have been much deceived by this temporary display of rational discourse, and it generally occurs that we are disposed to form a hasty conclusion, in proportion to the paucity of our knowledge on any given subject:—most minds feel more invited to indulge in the convenience of a syllogism, than to undergo the toil of induction. Although an insane person may be calm and apparently rational at the beginning of an interview, yet when least expected his disorder breaks forth, and in many instances, there seems to be no cause for this conversion from apparent sanity to evident derangement. In the commencement of the conversation the lunatic shall evince a healthy perception of existing objects, and institute a just admeasurement of the subject under contemplation; or in other words shall reason correctly; if he be placed in the society of other madmen he is able to detect the folly and aberration from reason which characterize their peculiar phantasies, and will often endeavour to convince them of the absurdity of their prevailing opinions, yet in a moment his mind launches into the regions of fiction, its admired clearness becomes obscured, and its seeming regularity exhibits a confused assemblage or violent distortion.—There is no intermediate condition which separates these states: and the transition very much resembles the last connected glimpses of our waking thoughts, followed by the abrupt creation of a dream.