KNOWLEDGE AND BELIEF. 301 



All knowledge of the truth depends on two different, 

 but intimately connected, groups of human physio- 

 logical functions : firstly, on the sense-impressions of 

 the object by means of sense-action, and, secondly, 

 on the combination of these impressions by an 

 association into presentations in the subject. The 

 instruments of sensation are the sense-organs (sensilla 

 or (Estheta) ; the instruments which form and link 

 together the presentations are the organs of thought 

 (phroneta). The latter are part of the central, and 

 the former part of the peripheral, nervous system — 

 that important and elaborate system of organs in the 

 higher animals which alone effects their entire 

 psychic activity. 



Man's sense-activity, which is the starting-point 

 of all knowledge, has been slowly and gradually 

 developed from that of his nearest mammal relatives, 

 the primates. The sense-organs are of substantially 

 the same construction throughout this highest animal 

 group, and their function takes place always according 

 to the same physical and chemical laws. They have 

 had the same historical development in all cases. In 

 the mammals, as in the case of all other animals, the 

 sensilla were originally parts of the skin ; the sensitive 

 cells of the epidermis are the sources of all the 

 different sense-organs, which have acquired their 

 specific energy by adaptation to different stimuli 

 (light, heat, sound, chemical action, etc.). The rod- 

 cells in the retina of the eye, the auditory cells in the 

 cochlea of the ear, the olfactory cells in the nose, and 

 the taste cells on the tongue, are all originally derived 

 from the simple, indifferent cells of the epidermis, 

 which cover the entire surface of the body. This 

 significant fact can be directly proved by observation 



