The different factors are somewhat arbitrarily illustrated in the accompanying diagram, the arrows indicating the physical stimulation and the conscious response: Of the six factors involved in the sensation, 1 and 2 are purely physical and belong to the science of acoustics; 3, 4, and 5 are physiological; 6 is conscious, or psychological. It is because they always involve the immediate presence of some physical object, that the sensation elements involved in ordinary perception are spoken of as immediate, or presentative, elements of knowledge. CLASSIFICATION OF SENSATIONS Our various sensations are usually divided into three classes as follows: 1. Sensations of the special senses, including: sight, sound, touch (including temperature), taste, and smell. 2. Motor, or muscular, sensations. 3. Organic sensations. =Sensations of the Special Senses.=--As a study of the five special senses has been made by the student-teacher under the heading of physiology, no attempt will be made to explain the structure of these organs. It must be noted, however, that not all senses are equally capable of distinguishing differences in quality. For example, it seems quite beyond our power to recall the tastes and odours of the various dishes of which we may have partaken at a banquet, while on the other hand we may recall distinctly the visual appearance of the room and the table. It is worthy of note, also, that in the case of smell, animals are usually much more discriminative than man. Certain of our senses are, therefore, much more intellectual than others. By this is meant that for purposes of distinguishing the objects themselves, and for providing the mind with available images as materials for further thought, our senses are by no means equally effective. Under this heading the special senses are classified as follows: Higher Intellectual Senses: sight, hearing, touch. Lower Intellectual Senses: taste and smell.