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powers of the mind had previously been rendered fully operative by means of those assistances which it usually receives from the bodily organs. Such instances as these, however they may at first appear, are extremely important. They furnish us with an appeal, not to mere speculations, but to fact. And it is only by checking undue speculation, and by continually recurring to facts, that our progress in this science will become sure, rapid, and delightful. 7 1 that
SENSATION AND PERCEPTION..
8 9. Sensation a simple mental state originating in the senses. In tracing the history of that portion of human thought which is of external origin, we have frequent occasion to make use of the words Sensation and Perception. The term SENSATION is not of so general a nature as to include every variety of mental state, but is limited to such as answer to a particular description. It does not appear that the usage of language would forbid our speaking of the feelings of warmth, and coldness, and hardness, as well as of the feelings of love, and benevolence, and anger, but it would clearly forbid our using the term SENSATION with an application equally extensive. Its application is not only limited, but is fixed with a considerable degree of precision.
Sensation, being a simple act or state of the mind, is unsusceptible of definition, and this is one of its characteristics. As this alone, however, would not separate it from many other mental states, it has this peculiarity to distinguish it, that it is immediately successive to a change in some organ of sense, or, at least, to a bodily change of some kind. But it is evident, that, in respect to numerous other feelings, this statement does not hold good. They are immediately subsequent, not to bodily impressions, but to other states of the soul itself. Hence it is, that
while we speak of the sensations of heat and cold, of hardness, of smoothness, roughness, and the like, we do not commonly apply this term to joy and sorrow, hatred and love, and other emotions and passions.
Ø 10. All sensation is properly and truly in the mind. Sensation is often regarded as something having a position, and as taking place in the body, and particularly in the organ of sense. The sensation of touch, as we seem to imagine, is in the hand, which is the organ of touch, and is not truly internal ; the hearing is in the ear, and the vision in the eye, and not in the soul. But all we can say with truth and on good grounds is, that the organs of sense are accessory to sensation and necessary to it; but the sensation or feeling itself is wholly in the mind. How often it is said the eye sees; but the proper language, if we look at the subject philosophically, is, that the soul sees; for the eye is only the organ, instrument, or minister of the soul in visual perceptions.
“A man,” says Dr Reid, “ cannot see the satellites of Jupiter but by a telescope. Does he conclude from this that it is the telescope that sees those stars? By no means; such a conclusion would be absurd.
It is no less absurd to conclude that it is the eye
that ear that hears. The telescope is an artificial organ of sight, but it sees not
is a natural organ of sight, by which we see; but the natural organ sees as little as the artificial.”
sees, or the
Ø 11. Sensations are not images or resemblances of objects. But while we are careful to assign sensations their true place in the mind, and to look upon what is outward in the body as merely the antecedents or cause of them, it is a matter of some consequence to guard against a danger directly the reverse of that which has been remarked on. We are apt to transfer to the sensation, considered as existing in the mind, some of those qualities which belong to the external object. But in point of fact, our sensations are by no means copies, pictures, or images of outward objects; nor are they representations of them in any material sense whatever; nor do they possess any of their qualities.
It is true, we often think it otherwise ; constantly occupied with external objects, when in the act of contemplation we retire within the mind, we unwarily carry with us the form and qualities of matter, and stamp its likeness on the thought itself. But the thought, whatev
may by the constitution of our nature be the sign of, has no form, and presents no image analogous to what are outwardly objects of touch and sight; nor has it form or image in any sense which we can conceive of. When, therefore, we have an idea of some object as round, we are not to infer, from the existence of the quality in the outward object, that the mental state is possessed of the same quality. When we think of anything as extended, it is not to be supposed that the thought itself has extension. When we behold and admire the varieties of colour, we are not at liberty to indulge the presumption that the inward feelings are painted over, and radiant with corresponding hues. There is nothing of the kind; and the admission of such a principle would lead to a multitude of errors.
$ 12. The connexion between the mental and physical change not capa.
ble of explanation. (I.) External bodies operate on the senses, before there is any affection of the mind, but it is not easy to say what the precise character and extent of this operation is. We know that some object capable of affecting the organ must be applied to it in some way either directly or indirectly, and it is a matter of knowledge also, that some change in the organ actually takes place; but further than this we are involved in uncertainty. All we can undertake to do at present is merely to make a statement of the facts, viz., the application of an external body, and some change in consequence of it in the
of (II.) Subsequently to the change in the organ, either at its extremity and outward developement or in the brain, with which it is connected, and of which it may be considered as making a part, a change in the mind or a new state of the mind immediately takes place. Here also we are limited to the mere statement of the fact We here touch upon one of those boundaries of the intellect which men are probably not destined to pass in the present life. We find ourselves unable to resolve and explain the connexion between mind and matter in this case, as we do in all others. All we know, and all we can state with confidence is, that a mental affection is immediately subsequent to an affection or change which is physical. Such is our nature, and such the appointment of Him who ordered it.
$ 13. Of the meaning and nature of perception. We next come to the subject of PERCEPTION, which is intimately connected with that of sensation. This term, like many others, admits of a considerable latitude in its application. In common language we are not only said to have the power of perceiving outward objects, but also of perceiving the agreement or disagreement in the acts of the mind itself. Accordingly, we perceive a tree in the forest or a ship at sea, and we also perceive that the whole is greater than a part, and that the three angles of a triangle are equal to two right angles. But what we have to say here does not concern internal perception, but merely that which relates to objects exterior to the mind.
Perception, using the term in its application to outward objects, differs from sensation as a whole does from a part; it embraces more. It may be defined, therefore, an affection or state of the mind which is immediately successive to certain affections of the organs of sense, and which is referred by us to something external as its cause.
$ 14. Perception makes us acquainted with a material world. It will be recollected, that the term SENSATION, when applied to the mind, expresses merely the state of the mind, without reference to anything external, which might be the cause of it, and that it is the name of a truly simple feeling. Perception, on the contrary, is the name of a complex mental state, including not merely the internal affection of the mind, but also a reference to the exterior cause. Sensation is wholly within; but Perception carries us, as it were, out of ourselves, and makes us acquainted with the world around us. It is especially by
means of this last power, that material nature, in all its varieties of form and beauty, is brought within the range of our inspection. If we had but sensation alone, there would still be form, and fragrance, and colour, and harmony of sound, but it would seem to be wholly inward. The mind would seem to constitute everything; we could know no other world, no other form of being. Perception prevents the possibility of such a mistake; it undeceives and dissipates the flattering notion, that all things are in the soul; it leads us to other existences, and, in particular, to the knowledge of the vast and complicated fabric of the material creation.
$ 15. Of the primary and secondary qualities of matter. From what has been said, it will be noticed that SENSATION implies the existence of an external material world as its cause, and that PERCEPTION implies the same existence both as cause and object. It is hardly necessary to say, that we are altogether ignorant of the subjective or real essence of matter. Our knowledge embraces merely its qualities or properties, and nothing more. Without proposing to enter into a minute examination of them, it will be proper to state here, that the qualities of material bodies have been ranked by writers under the two heads of Primary and Secondary.
The PRIMARY QUALITIES are known by being essential to the existence of all bodies. They are extension, figure, divisibility, and solidity; and some writers have included motion. They are called PRIMARY for the reason already distinctly referred to, that all men embrace them in the notions which they form of matter, and that they are essential to its existence. All bodies have extension, all bodies have figure, all are capable of division, all possess the attribute of solidity.
By SOLIDITY in bodies (perhaps some would prefer the term RESISTANCE) is to be understood that quality by which a body hinders the approach of others between which it is interposed. In this sense even water, and all other fluids are solid. If particles of water could be prevented from separating, they would oppose so great resistance, that it would be impossible for any two bodies between