30 THE CAUSATION OF DISEASE. 



included under this mental E. These agencies are not im- 

 material : they are essentially physical. For mind can only 

 be affected from without by impressions made upon the senses, 

 and our sense organs are stimulated by physical forces, in a 

 manner more or less perfectly understood — the retina by ether- 

 waves, the ear by waves of air, the skin by a material contact 

 more or less massive. Thus the active physical world operates 

 upon the material brain, giving rise to a series of changes, 

 which are the physical accompaniments of mental states. Con- 

 cerning the nature of the connection between the physical and 

 the accompanying mental chain, I say nothing. All we know, 

 but that we know most certainly, is that while we inhabit 

 u this muddy vesture," there is such a connection, and that for 

 practical purposes it is both useless and unscientific to treat 

 of mind as a separate entity ; we had best regard it, with 

 Bain, as a double-faced unity. 



It is thus through a physical medium that the animate and in- 

 animate worlds (in their purely physical aspect) affect our mental 

 being, as when we observe the forms, colour and movements of 

 objects, animate and inanimate, about us. Thus also are we 

 affected by the mental world outside us — namely, are brought 

 into communion with the minds of others, living and dead. 

 Our mental education consists in a series of physical influences 

 coming from without. When a boy is sent to school it is with 

 the object of bringing such influences to bear upon his brain. 

 His education consists in the skilful regulation of the exter- 

 nal-mental-E. In this way his nerve centres are moulded 

 in divers subtle ways, and thus is built up a character good 

 or bad, and an intellectual being great or small, according to 

 the nature of the influences brought to bear upon the indi- 

 vidual, and the peculiar plasticity, as determined by the laws of 

 heredity, of the nervous matter operated upon. 



These observations on external-body-E render it evident 

 that the environing influences of living organisms are not 

 simple, but complex in the extreme. The infinite variety and 

 complexity of the conditions which go to make up the external- 

 body-E, can be best appreciated by bearing in mind that every 

 separate species of plant and animal has a distinct and specific 

 extern al-E ; nay, more than this, it is utterly impossible, as we 



