xii INTRODUCTION. 
posited in points or centres, and each centre acting 
on the others, when we review the nervous system 
in its relations with consciousness we pass from the 
mere mutual relations of such centres, and find both 
mind acting on matter, and matter acting on mind. 
Those who deny the evidence of spirit existing inde- 
pendentof matter point to the permanence of energy 
in the material world. They argue that the mind 
is affected by physical stimuli, and that no mental 
operation takes place without corresponding physi- 
cal action. They assume that, were consciousness 
beyond the circle of physica! changes, it would re- 
main unaffected by them, instead of mind acting on 
body, and body on mind, as is the case ; but if it be 
within the circle of physical change then, they argue, 
it is itself physical, or at least we have no right to 
suppose that it can exist independent of matter. 
From the dogma that mind cannot exist independ- 
ent of matter, one can easily see how simple is the 
passage to some such formula as that mind is an 
evolution of matter. But to extract a clear mean- 
ing from such a formula is not so easy: it is the 
endowing of matter with properties demonstrated in 
no physical nor chemical laboratory. Thus when 
in the passage quoted at p. 23, Hackel speaks of 
spirit as a function of force he uses a form of words 
which conveys no meaning. Had he said that force 
isa function of spirit he might have reached a truly 
“ monistic” philosophy and thrown the mind back on 
