By intensity of idea, I presume is meant, that the mind is more strongly fixed on, or more frequently recurs to, a certain set of ideas, than when it is in a healthy state. But this definition applies equally to mania, for we every day see the most furious maniacs suddenly sink into a profound melancholy; and the most depressed, and miserable objects, become violent and raving. We have patients in Bethlem Hospital, whose lives are divided between furious, and melancholic paroxisms; and who, under both states, retain the same set of ideas. Insanity may, in my opinion, be defined to be _an incorrect association of familiar ideas, which is independent of the prejudices of education, and is always accompanied with implicit belief, and generally with either violent or depressing passions_. It appears to me necessary, that the ideas incorrectly associated, should be _familiar_, because we can hardly be said to have our ideas deranged upon subjects, concerning which we have little or no information. A peasant, who had heard that superior comforts of life, with fewer exertions, were to be obtained by emigrating to America, might saddle his beast with an intention of riding thither on horse-back, without any other imputation than that of ignorance; but if an old and experienced navigator, were to propose a similar mode of conveyance, I should have little hesitation in concluding him insane. Respecting the prejudices of education, it may be observed, that in our childhood, and before we are able to form a true, and accurate judgment of things, we have impressed upon our minds, a number of ideas which are ridiculous; but which were the received opinions of the place in which we then lived, and of the people who inculcated them; such is the belief in the powers of witchcraft, and in ghosts, and superstitions of every denomination, which grasp strongly upon the mind and seduce its credulity. There are many honest men in this kingdom who would not sleep quietly, if a vessel filled with quicksilver were to be brought into their houses; they would perhaps feel alarmed for the chastity of their wives and daughters; and this, because they had been taught to consider that many strange and unaccountable properties are attached to that metal. If a lecturer on chemistry were to exhibit the same fears, there could be no doubt that he laboured under a disorder of intellect, because the properties of mercury would be known to him, and his alarms would arise from incorrectly associating ideas of danger, with a substance, which in that state is innoxious, and whose properties come within the sphere of his knowledge.