Usually, however, a wider range has been given to the term, and the faculty thereby denoted. It has been made to comprehend any mental process by which we refer a specific sensation to something external as its producing cause. It is thus employed by Reid and Stewart, and such has been in fact the prevalent use of the term. According to this, when we experience the sensation of fragrance, and refer that sensation to the presence of a rose, or the sensation of sound, and refer it to the stroke of a bell, or a passing carriage, we exercise the faculty of perception. Evidently, however, our knowledge in these cases is merely a matter of inference, of judgment, not of immediate direct perception, not in fact of perception at all. All that we properly perceive in such a case, all that we are directly conscious of is the fragrance or the sound. That these are produced by the rose and the bell is not perceived, but only conceived, inferred—known, if at all, only by the aid of previous experience.
Sensation as distinguished from Perception.—According to the view now presented, sensation, as distinguished from perception, is the simple feeling which results from a certain affection of the organism. It is known to us merely as feeling. Perception takes cognizance of the feeling as an affection of the organism, and also of the organism as thus affected, and consequently as external to the me, extended, having parts, etc. It apprehends also objects external to the organism itself limiting and affecting its movements. Sensation is the indispensable condition of perception. If there were no sensation, there would be no perception. The one does not precede, however, and the other follow in order of time, but the one being given, the other is given along with it. The two do not, however, coexist in equal strength, but in the relation, as stated by Hamilton, of inverse ratio; that is, beyond a certain point, the stronger the sensation, the weaker the perception, and vice versâ.
Sensation as an Affection of the Mind.—It has been common to speak of sensation as lying wholly in the mind. Primarily, however, it is an affection of the nervous organism, and through that organism, as thus affected, an impression is made on the mind. If it were not for the mind present with the organism, and susceptible of impression from it, and thus cognizant of changes in it, the same changes might be produced in the organism as now, but we should be entirely unconscious of and insensible to them. In certain states of the system this actually happens, as in sound sleep, the magnetic state, the state produced by certain medicinal agents as ether, chloroform, opium, and the intoxicating drugs of the East. In those cases, the connection between the mind and the nervous organism seems to be in some manner interrupted or suspended, and consequently there is for the time no sensation. The nerves may be irritated, divided even, and still no pain is felt.
It is not true, however, that the sensation is wholly in the mind. It is in the living animated organism, as pervaded by the mind or spiritual principle, mysteriously present in every part of that organism, and cognizant of its changes; and neither the body alone, nor the mind alone, can be said to possess this faculty, but the two united in that complex mysterious unity which constitutes our present being.
§ III.—Analysis and Classification of the Qualities of Bodies.
Difference of Qualities.—The qualities of bodies as known to us through sensation and perception are many and various. On examination, a difference strikes us as existing among these qualities, which admits of being made the basis of classification. Some of them are qualities which strike us at once as essential to the very existence of matter, at least in our notion of it, so that we cannot in thought divest it of these qualities, and still retain our conception of matter. Others are not of this nature. Extension, divisibility, size, figure, situation, and some others, are of the former class. If matter exists at all, it must, according to our own conceptions, possess these qualities. We cannot think them away from it, and leave matter still existing. But we can conceive of matter as destitute of color, flavor, savor, heat, cold, weight, sound, hardness, etc. These are contingent and accidental properties not necessary to its existence.
How named and distinguished.—Philosophers have called the former class primary, the latter secondary qualities. The former are known à priori, the latter by experience. The former are known as qualities, in themselves, the latter only through the affections of our senses.
The primary qualities then have these characteristics:
1. They are essential to the very existence of matter, at least in our conception.