INTRODUCTION. ie 
ence is made to the doctrine of sensation put 
forward in the memoir “ On the Physical Relations 
of Consciousness and the Seat of Sensation.” That 
doctrine may not have received much attention ; 
but it is in the position of remaining unassailed, 
while the old doctrine is unsatisfactory to some of the 
most competent judges, and remains undefended 
from the objections here brought against it. 
With regard to the fifth article of the series, it 
need only be said that the connection of the theory 
of cell-life with that of life in every other aspect is 
too close to require more than mention here ; while 
to one who believes in life within life and in the 
unity of cause in the order of events in each, cell-life 
is especially interesting as the simplest of a series of 
which the most complex known is the life-evolution 
on the face of the earth. 
Every one is familiar by this time with the re- 
luctance of certain physicists and naturalists to take 
into consideration even the possibility of such an 
element as spirit being necessary for the construc- 
tion of a rational philosophy of nature. The 
wonderful advances in physics, affording sure foot- 
ing for further progress within that domain, and 
giving play for speculations formerly inconceivable, 
may lead some physicists to overleap those strict 
fences the observance of which has secured the ad- 
vance of their favourite studies, and they may 
dream that the physical is the only world. So also 
