2 1 8 Heredity. 



Then, passing from the abstract to the concrete, from theory to 

 experience, we shall strive to show that every mental state implies 

 a corresponding physical state. 



Thence we shall draw the conclusion that an habitual mental 

 state, such as psychological heredity, must have as its condition 

 an habitual physical state, such as physiological heredity. 



In the seventeenth century, the question of the ' union of soul 

 and body ' was put in a form which rendered it insoluble. It was 

 a problem of metaphysics. There were held to be two substances, 

 body and mind ; between the two an abyss. All their character- 

 istics were opposed ; then, as was to have been expected, it was 

 found impossible to join together again what had been so 

 thoroughly sundered. 



Since the time when the progress of physiology showed that the 

 nervous system is the physical condition of moral phenomena, and 

 that every variation in the one is coupled with a variation in the 

 other, researches into the correlation of the physical and the 

 moral have had a firm basis, for the reason that it has been 

 possible to rest them on a something which is the body, even 

 while it is the instrument of the soul Thus is explained the 

 invasion, ever widening since the seventeenth century, of neurology 

 into psychology. 



Nor is this all. A further step in progress, which now appears 

 to have been made by all partisans of experimental inquiry, 

 consists in substituting for the metaphysical the experimental 

 point of view, and for the antithesis of two substances the anti- 

 thesis of two groups of phenomena. Hence the problem is no 

 longer the relations between body and soul, but the relations 

 between a group of phenomena pertaining to the unit which we 

 call life, and the group pertaining to the unit called the ego. It is 

 true that this way of putting the question simplifies it only by 

 making it insoluble ; for when we restrict ourselves to experience, 

 we renounce in advance all ultimate and absolute reason. But as 

 the experimental sciences are strictly speaking made up of two 

 things facts and hypotheses and as the human mind has an in- 

 vincible tendency always to sacrifice the facts to the hypotheses, 

 we, if we resist this tendency, run the risk of throwing away 

 the booty for its shadow. 



