In preceding chapters we have seen that in the phase of mental activity in which the Intellect is concerned, the processes of which are known as "Thought" in the narrower sense of the term, there are several stages or steps involving the use of several faculties of the mind. The first of these steps or stages is called _Perception_. Many persons confuse the idea of Sensation and Perception, but there is a clear distinction between them. Sensations arise from nerve action--from the stimulation of nerve substance--which gives rise to a peculiar effect upon the brain, which results in an elementary form of consciousness. An authority says: "Sensation is the peculiar property of the nervous system in a state of activity, by which impressions are conveyed to the brain or sensorium. When an impression is made upon any portion of the bodily surface by contact, heat, electricity, light, or any other agent, the mind is rendered conscious of this by sensation. In the process there are three stages--reception of the impression at the end of the sensory nerve, the conduction of it along the nerve trunk to the sensorium, and _the change it excites in the sensorium itself, through which is produced sensation_." Just why and how this nerve action is translated into consciousness of an elementary kind, science is unable to explain. Our knowledge is based in a great part, or entirely, upon impressions which have been received over the channel of the senses--sensations of sight, hearing, tasting, smelling and touch. Many authorities hold that all of the five senses are modifications of the sense of touch, or feeling; as for instance, the impression upon the organs of sight is really in the nature of a delicate touch or feeling of the light-waves as they come in contact with the nerves of vision, etc. But, although sensations give us the raw materials of thought, so to speak, they are not _knowledge_ in themselves. Knowledge arises from the operation of Perception upon this raw material of Sensation. But yet, Sensation plays a most active part in the presentation of the raw material for the Perceptive faculties, and must not be regarded as merely a physiological process. It may be said to be the connecting link between the physical and the mental activities. As Ziehen says: "It follows that the constitution of the nervous system is an essential factor in determining the quality of sensation. This fact reveals the obvious error of former centuries, first refuted by Locke, though still shared by naive thought today, that the objects about us themselves are colored, warm, cold, etc. As external to our consciousness, we can only assume matter, vibrating with molecular motion and permeated by vibrating particles of ether. The nervous apparatus selects only certain motions of matter or of ether, which they transform into that form of nerve excitation with which they are familiar. It is only this nerve excitation that we perceive as red, warm or hard."