All the operations in the mind are the effects of motions previously excited in the brain, and every idea and thought appears to depend upon a motion peculiar to itself. In a sound state of the mind, these motions are regular, and succeed impressions upon the brain with the same certainty and uniformity that perceptions succeed impressions upon the senses in their sound state.
In inquiring into the causes of the diseases of the mind, and the remedies that are proper to relieve them, I shall employ the term derangement, to signify the diseases of all the faculties of the mind.
As the understanding occupies the highest rank of those faculties, and as it is most frequently the seat of derangement, I shall begin by considering the causes, and all the states and forms of its diseases.
By derangement in the understanding, I mean every departure of the mind in its perceptions, judgments, and reasonings, from its natural and habitual order; accompanied with corresponding actions. It differs from delirium, whether acute, or chronic, in being accompanied with a departure from habitual order, in incoherent conduct, as well
