SECTION I.

OF SENSATION AND PERCEPTION.

We know nothing of perception except the fact that cortain impressions made upon the organs of sense convey to the mind a knowledge of the properties of external things. Some of the older speculations on this subject have already been referred to. In these the mind was compared to a camera obscura, and the transmission of the forms or images of things to it from the organs of sense was explained by the motion of the animal spirits, or the nervous fluid, or by vibrations in the substance of the nerves. All such speculations are now dismissed from the investigation, being considered as attempts to penetrate into mysteries which are beyond the reach of the human faculties, and consequently not the legitimate objects of a philosophical inquiry.

Our first knowledge of the existence and properties of the material world is evidently of a complex nature. It seems to arise from the combined action of several senses, conveying to us the general notion of certain essences which are solid and extended, or possessed of those properties which characterize material things. Without this general knowledge previously acquired, our various senses acting individually could convey to us no definite notion of the properties of external things. A smell, that is, a mere odor, for example, might be perceived by us, but would convey nothing more than the sensation simply. It could not communicate the impression of this being a property of an external body, until we had previously acquired a knowledge of the existence of that body, and had come by observation to associate the sensation with the body from which it proceeds. The same holds true of the other senses; and we are thus led at the very first step of our inquiry to a complicated process of mind without which our mere sensations could convey to us no definite knowledge.

Having thus acquired a knowledge of the existence and

Former theories. How now considered? First knowledge-how obtained? Suc ceeding steps-what?

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general properties of material things, we next derive from our various senses a knowledge of their more minute characters. These are generally divided into primary and secondary. The primary qualities of material things are such as are essential, and must at all times belong to matter; such as solidity and extension. These properties necessarily convey to us a conviction of something existing out of the mind, and distinct from its own sensations. The secondary qualities, again, are color, temperature, smell, taste, &c. These are not essential properties of matter, but qualities producing sensations in a sentient being; they may or they may not belong to any particular body, or they may be attached to it at one time and not at another. Hence they convey to us primarily no definite notion in regard to the existence or properties of external things, except, as Mr. Stewart expresses it, as the unknown cause of a known sensation." One of the quibbles or paradoxes of the scholastic philosophy was, denying the real existence of these secondary qualities of matter. Every one is familiar with the humorous account given in the "Guardian" of the attainments of a youth from college, and his display of them when on a visit to lady Lizard, his mother. "When the girls were sorting a set of knots he would demonstrate to them that all the ribands were of the same color, or rather of no color at all. My lady Lizard herself, though she was not a little pleased with her son's improvement, was one day almost angry with him; for, having accidentally burnt her fingers as she was lighting the lamp for her teapot, in the midst of her anguish Jack laid hold of the opportunity to instruct her that there is no such thing as heat in the fire.' Such speculations, which were at one time common in the schools of philosophy, had their origin wholly in an abuse of terms. The term heat, for example, has two meanings, which are quite distinct from each other. It means a sensation produced in a sentient being, and in this sense it may be said with truth that there is no heat in fire; but it means also a quality in material substances capable of producing this sensation, and it is in this sense that we speak of heat as a property of matter.

Classification of qualities. Definitions. Extract given in the Guardian.

Notwithstanding this explanation of the different senses in which the word heat is used, many persons find it difficult to understand that there is any sense in which it can be said with truth that there is no heat in fire. But a little reflection will make it plain.

If a man puts his hand among coals he feels a burning, painful sensation, which we call heat. Now when it is said there is no heat in fire, the meaning is that there is no such burning, painful sensation. And certainly no one can suppose that there is. There cannot be suffering in the fire, or even any feeling of warmth, or sensation of any kind; and it is in this sense alone that the word is used, when the exis tence of heat in the fire is denied. So with all the other secondary qualities. Smells, tastes, sounds, &c. are all feelings in us. The external objects themselves cannot have these feelings, or any other. They have some peculiarity or property which excites these feelings in us, but not the feelings or sensations themselves. 5

The process by which we acquire a knowledge of external things is usually divided into two stages, namely, sensation and perception; the former implying the corporeal, the latter the mental part of it. Others apply the term perception to both; and, according to Dr. Brown, sensation ist the simple impression made upon the organs of sense; perception is an association formed between this impression and an external substance which we have ascertained to be concerned in producing it. Our senses, by which this knowledge is acquired, are generally reckoned five,-viz: sight, hearing, taste, smell, and touch. Dr. Brown proposes to add our muscular frame, and apparently with good reason; for there seems ground for believing that it is by resistance to muscular action that we acquire the notion of solidity, and that this could not be acquired by touch alone.

Our first impression of the existence and solidity of material objects, then, seems to be derived from touch combined with muscular resistance; and at the same time we acquire the knowledge of temperature, roughness or smoothness, &c. There has been some difference of opinion in regard to the manner in which we acquire the notion of extension, including figure and magnitude. It is evident that it cannot be acquired from touch alone; but it may be acquired from touch combined with muscular motion, as when we move the hand over the surface of a body. This, however, includes also the idea of time,-for our notion of the extent of a surface when the hand moves over it is very

Explanation. Secondary qualities; their nature? Distinction between sensation and perception. Number of the senses. First notions-how obtained ?

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much influenced by the velocity with which the motion is made. Hence time has been supposed by some to be one of our very earliest impressions, and antecedent even to the notion of extension or space. It is probable, however, that the notion of extension may also be acquired in a more simple manner from the combined operation of touch and vision. If this opinion be correct, it will follow that our first knowledge of the existence and essential properties of material things is derived from the combined operation of sight, touch, and muscular action.

With regard to all our senses, however, the truth seems to be, that the first notions conveyed by them are of a very limited and imperfect kind; and that our real knowledge is acquired only after considerable observation and experience, in the course of which the impressions of one sense are corrected and assisted by those of others, and by a process of mind acting upon the whole. The primary objects of vision, for example, seem to be simply light or color and expansion. But the judgments which we are in the daily habit of forming upon vision are of a much more extensive kind, embracing also distance, magnitude, and what has been called tangible figure, such as the figure of a cube or a sphere. This last, it is evident, cannot be considered as a primary object of vision, but as entirely the result of experience derived from the sense of touch; for we never could have formed any conception of the figure of a cube or a sphere by vision alone. Distance and magnitude, also, are evidently not the primary objects of vision; for persons who have been suddenly cured of congenital blindness, by the operation for cataract, have no conception of the distance or magnitude of objects; they perceive only simple expansion of surface with color. Our judgment of distance and magnitude by vision, therefore, is an acquired habit, founded upon the knowledge which we have received by other means of the properties of the objects. Accordingly, it is familiar to every one, that we have no idea of the distance of an object, except we have some notion of its magnitude; nor, on the other hand, of' its magnitude, except we have some knowledge of its distance. The application of

First notions derived from the senses. Primary objects of vision? Ideas of distance and magnitude-how obtained? Connection of these ideas.

this principle is also familiar in perspective drawing, in which the diminished size of known objects is made to convey the notion of distance. On the same principle, known objects seen through a telescope do not appear to be magnified, but to be brought nearer. In the same manner with regard to sounds; we have no idea of their intensity, except we have some notion of their distance, and vice versa. A given degree of sound, for example, if we believed it to have been produced in the next room, we might conclude to proceed from the fall of some trifling body; but if we supposed it to be at the distance of several miles, we should immediately conclude that it proceeded from a tremendous explosion.

In regard to certain small distances, however, there is a power of judging by sight alone; and it appears to arise out of the degree of inclination which is given to the axis of vision in directing the two eyes to the object. Thus, in snuffing a candle, or carrying the finger to a small object within arm's length, it will be found that we are very apt to miss it if we look with one eye only, but can touch it with unerring certainty when both eyes are directed to it.

This experiment may be easily tried. Hold some small object, a lead pencil for instance, with the point upwards at the distance of about a foot from the eye. Then, with one eye closed, endeavor to bring the end of the finger down exactly upon the point of the pencil. It will be found quite difficult to do it exactly, though with both eyes open it will be easy.

It appears to be on the same principle that we enjoy in a greater degree the deception produced by a painting, when we look at it with one eye, especially if we also look through a tube. By the former we cut off the means of correcting the illusion by the direction of the axis of vision; and by the latter we remove the influence of all neighboring objects. It is impossible to determine the precise distance to which we can extend this power of judging of distance by the inclination of the axis of vision, but it does not appear to be great; and in regard to all greater distances, the judg ment by vision is evidently an acquired habit, arising out of such a mental exercise as has now been referred to.

Intensity and distance of sounds. Small distances judged of by sight alone. Experi ment with paintings.