In this work, the aim is to introduce no more of mental analysis than is needed for its main object. What is here introduced is not claimed as a complete presentation of all the mental phenomena.
CHAPTER IX.
SENSATION AND PERCEPTION.
As there is no distinction between sensation and perception except in the fact that one is attended with the belief of a cause and the other is not, they will be treated of together.
The mind of man is an immaterial existence, confined in its operations by the body it inhabits, and depending upon the construction and modifications of this envelope for much of its happiness or suffering.
The exercise of the imagination, when the eyes are closed and the body at rest, will probably give us the best idea of what is the nature of spiritual existence when disconnected with matter. It is one of the offices of our bodily system to retain the spirit in its operations in one particular place, so that ordinarily it can have direct communion with no other mind which is not in the same place. Whether this is the case with mere spiritual existence is a question for conjecture, and not for any rational decision.
While the spirit of man is resident in its material frame, it is furnished with facilities of communication with other minds, and with organs which fit it to receive suffering or enjoyment from the material objects by which it is surrounded. These organs of communication are the several senses. They consist of expansions of the substance of which the brain is formed, which, descending to the body through the spinal bone of the back, are thence sent out in thousands of ramifications over the whole system. Those branches which enter the eyes, and are spread over the interior back part of this organ, are called the optic nerve. Whenever the particles of light enter the eye, they strike the optic nerve, and produce the sensation which is called sight. Those branches which are spread over the tongue are the organ of taste. Those that are extended through the cavities of the nostrils are called the olfactory nerves. When the small particles of matter that escape from odoriferous bodies come in contact with these nerves, they produce the sensation of smell.
The nerves that constitute the organ of hearing are extended over the cavity of the ear behind the tympanum, or ear-drum. This cavity is filled with a liquid, and when the drum of the ear is caused to vibrate by the air which is set in motion by sonorous bodies, it produces undulations of this liquid upon these nerves, and thus the sensation of sound is produced. By the expansion of other nerves, the sense of feeling is extended all over the body, excepting the nails and the hair. It is by the action of matter, in its different forms, on these several senses, that the mind obtains ideas, and that ideas are imparted from one mind to another.
Perception never takes place unless some material object makes an impression upon one of the senses. In the case of the eye, the ear, and the nostrils, the object which is regarded as the cause of the sensation does not come immediately in contact with the organs of sense. When we see a body, we consider it as the cause of that perception; but it is not the body that comes in contact with the organ of sight, but merely the particles of light reflected from that body. In the case of smell, the fragrant body is regarded as the cause of the sensation; but that which acts on the sense is the material particles of perfume which flow from that body.
Thus, also, with hearing. We consider the sonorous body as the cause; but the sensation is produced through the medium of the air, which affects the drum of the ear. But in the case of taste and touch, the body which is regarded by the mind as the cause must come in contact with the nerves of the tongue or the body to produce the sensation.