and the object presses against us; and the degree of firmness with which it resists us constitutes the degree of its hardness. The result of the physicist's examination of the external world is to change the ordinary view of it profoundly. He resolves its apparent brightness into vibrations, its sounds into waves in the air, its odors. and tastes into an emission of minute particles, and its hardness into resistance. Already the world of our naïve minds has undergone a reconstruction that results in an external world very different from what we supposed it to be. 2. THE PHYSIOLOGICAL VIEW The physiologist at this point takes up the problem of remodeling the world, and carries it a step farther. He dissects the nervous apparatus of sensation and discloses its delicate mechanism. Fine threads run from the various external sense organs to the sensory centers of the brain. The tympanum of the ear vibrates in unison with the air waves that in turn have been caused by the vibrating body. These vibrations of the tympanum are propagated through the complex apparatus of the internal ear to the auditory nerve, which transmits some kind of influence to the brain, producing in it an excitation on occasion of which the mind experiences sound. The auditory nerve does not transmit mechanical vibrations similar to those which reach the tympanum of the ear, but some other mode of activity of unknown Similar nature. This unknown influence is again unlike the vibrations of the sonorous body and wholly different from the experience of sound in the mind. statements apply to the optic nerves, transmitting the shocks of the ether vibrations upon the retina of the eye to the optic tract in the brain, and to the nerves transmitting the excitations of odor, taste, and touch from their sense organs to their respective brain centers. The nature of the unknown influence thus transmitted along the nerves is unlike the external cause of the sensation as it arrives at the sense organ, unlike the excitation of this organ itself, and wholly different from the sensation. Thus the nervous system interposes between the outer sense excitation and the inner mental experience links that are different from both. If sound consists of pulsations of certain wave lengths and frequency in the air, such pulsations do not reach the brain over the auditory nerve. If colors consist of certain vibrations of infinitesimal wave lengths and enormous rapidity in the ether, there are no such vibrations in the brain, for they cannot be transmitted over the optic nerves. Similarly, there are no sweet or odorous particles in the brain, for they cannot be transmitted along the nerves. If touch consists in pressure on the external ends of the nerves, there is no such pressure on the inner ends of the nerves, as they terminate in the brain. Thus the changes taking place in the brain are of a different nature from those taking place in the external world. If the external world consists of an extended reality that is luminous, sonorous, odorous, sapid, and resistant, such extended reality cannot be brought into the brain, for it is separated from the brain by lines of communication that cannot transmit such modes of reality. The physicist and the physiologist have combined to prove such transmission impossible. 3. THE PSYCHOLOGICAL VIEW The psychologist now appears on the scene and takes up the thread of investigation at its inner end. He studies the nature of sensation itself. At once he points out the fact that sensation is a mental experience totally different in nature from anything in the external world. It is a conscious state of the soul, and as such cannot be compared with anything material. The two things are so different in nature that there is no common ground or element in them. The one is a state of thinking and feeling, and the other is an unthinking, insensate substance. All our sensations are mental states. Sound is a state of mind in which we have a certain peculiar mental experience that cannot be described and can only be felt. Light is a state of mind in which we experience a sense of brightness. So taste, odor, and touch are mental states in which the soul is conscious of certain peculiar experiences. It is impossible to resolve a sensation into anything else than a mental state, or to think of it in the terms of material existence. A sensation cannot be round or square, bright or dark, sonor ous or silent, sweet or bitter, hard or soft, in itself, because it is not an extended material substance, but a mental affection, and mind does not and cannot have material qualities; it is sui generis, a kind of reality that is apart from material reality and refuses to mix with it. Sensation, then, has its seat in the mind and is never found outside of it. This is an elementary fact in psychology, but it is a fundamental fact with far-reaching consequences in metaphysics, and it may be well to illustrate it at some length in order that it may be seen clearly and grasped firmly. The fact is open to our immediate introspection. When we are hearing a sound or seeing a light, we are not aware of any vibrations in the air or in the ether, and may not have so much as heard of these things; yet the physicists tell us that these vibrations are all that is in the external world as the cause of our sensations of sound and light. We need only pay attention to what is going on in our minds to perceive that our sensations are wholly subjective. It is thus a mere truism, but one that needs to be emphasized in this connection, that there is no sensation of sound in a sonorous body, or of light in a luminous body, or of sweetness in sugar, or of odor in a rose, or of hardness in a stone. These sensations are in every instance states of experience in us caused by some action upon us by these things. There is no sound when a bell is ringing or when Niagara is falling, and no sentient mind is present to hear these things: all that Red and is going on in them is a state of motion, and motion cannot hear. The bell is not conscious and hears no sound. Niagara is only a mass of moving water and cannot hear its own fall. There is no redness in the rose, or green in the grass, or blue in the sky. green and blue are sensations in us, and have no existence or meaning outside of a sentient mind. The sun is not bright; it is only in a state of motion, and motion cannot be described as either bright or dark. The motion of the sun is transmitted through the ether to our eyes, and reaches the brain as some kind of action which is the exciting cause of the sensation of light in our minds; until that sensation is experienced, there is no light, but only motion. Of course we speak of the sun as being bright, and this use of language is proper and inevitable. But so also do we speak of the sun as rising and setting, and yet we know that this is only an appearance, and not the reality. Light is something that cannot exist in the sun, because the sun is not sentient and cannot experience any sensation of light; and light cannot be transmitted to us through the ether and the optic nerves, because these things cannot experience any sensation. There may be motion in the form of vibrations in the sun and in the ether, but there can be no light until the mind is affected with this sensation. And so it is with all the other sensations. Sugar is not sweet, because it cannot feel sweetness; it only has certain chemical powers by which it can act upon the tongue. We call it sweet by a metaphorical use of |