78 POPULAR SCIENCE MONTHLY. 



past systems of philosophy, perhaps all possible systems, have been 

 thrown back into the realm of literature, of poetry, no longer controlling 

 the life of action, which rests on fact. 



This conflict of tendencies in the individual has become a conflict 

 among individuals as each is governed by a dominant impulse. The 

 cause of tradition becomes that of theology; — for men have always 

 claimed a religious sanction for their own individual bit of cosmic 

 philosophy. Just as each man in his secret heart, the center of his 

 own universe, feels himself in some degree the subject of the favor 

 of the mysterious unseen powers, so does society in all ages find a mystic 

 or divine warrant for its own attitude towards life and action, what- 

 ever that may be. 



The nervous system of man, inherited from that of the lower ani- 

 mals, may be regarded as primarily a means of making locomotion 

 safe. The reflex action of the nerve center is the type of all mental 

 processes. The sensorium, or central ganglion, receives impressions 

 from the external world representing, in a way, various phases of 

 reality. The brain has no source of knowledge other than sensation. 

 All human knowledge comes through human experience. The brain, 

 sitting in darkness, has the primary function of converting sensory 

 impressions into impulses to action. To this end the motor nerves 

 carry impulses outward to the muscles. The higher function of nerve 

 action, which we call the intellect, as distinguished from simple reflex 

 action and from instinct, is the choice among different responses to the 

 stimulus of external realities. As conditions of life become more 

 complex, the demands of external realities become more exacting. It 

 is the function of the intellect to consider and of the mind to choose. 

 The development of the mind causes and permits complexity in external 

 relations. Safety in life depends on choosing the right response to 

 external stimulus. Wrong choice leads to failure or to death. 



From the demands of natural selection results the intense prac- 

 ticality of the mental processes. Our senses tell us the truth as to 

 external nature, in so far as such phases of reality have been essential 

 to the life of our ancestors. To a degree, they must have seen ' things 

 as they really are/ else they should not have lived to continue the gen- 

 eration. Our own individual ancestors through all the ages have been 

 creatures of adequate accuracy of sensation and of adequate power of 

 thought. Were it not so they could not have coped with their environ- 

 ment. The sensations which their brains translated into action con- 

 tained enough of absolute reality to make action safe. That our own 

 ordinary sensations and our own inductions from them are truthful 

 in their essentials, is proved by the fact that we have thus far safely 

 trusted them. Science differs from common sense mainly in the per- 

 fection of its tools. That the instruments of precision used in science 

 give us further phases of reality is shown by the fact that we can trust 



