
THE PRINCIPLE OF VITAL AFFINITY. 343 
them, at variance with the laws that regulate the changes of inorganic matter. 
and requiring to be investigated by a separate induction of facts, must be ad- 
mitted by all; and is indeed the only reason we can give for treating Physiology, 
and the branches of knowledge dependent on it, as a separate science; and this 
being so, it belongs to the very elements of the science to determine what are 
the portions of the history of living bodies which come under this category. 
I have always held in high respect the aphorism of HEBERDEN, which Dr 
GREGORY used to recommend to the special attention of his pupils, that the great 
desideratum in medical science is the detection of the Vital Principle, by which 
all that goes on in the living body is regulated and governed; but I have always 
thought likewise, that the object of this investigation is rightly limited by Dr 
Prout, when he says that we should inquire, ‘‘ not what the vital principle or 
vital power is, but what it does.” In fact, in all the sciences, we can acknow- 
ledge only one principle and one Power, as the origin of all the phenomena that 
we investigate; and when we use these terms in reference to living beings,—when 
we say that we inquire how the vital principle acts,—we use the term only as a 
convenient and simple expression for an investigation of the laws according to 
which the Divine power acts, in regulating the changes which are continually 
taking place in the last and noblest of the works of creation, and which differ 
from the changes that we see around us in other departments of nature. 
This precise and definite object of all physiological researches—the deter- 
mination of the laws that are peculiar to the science—has always attracted the 
attention of physiologists, but has not always been placed in the proper point of 
view ; and the common error in this, as in other sciences, has been, to regard the 
laws of nature as simpler than they really are, and to stretch a principle, ascer- 
tained as to one set of phenomena, in the hope that it would be found sufficient 
to embrace many more. Thus it was easily observed that the phenomena 
of sensation and thought, and the visible motions in animals, were quite peculiar 
to them; and when it was ascertained that the first of these, and that a large 
portion of the latter (viz., all voluntary motions), depend on the living state of 
the nervous system, it was hastily concluded that all the phenomena peculiar to 
animal bodies, depend on their Nervous System. This is illustrated by the title 
of one of the chapters in GrEGoRY’s “ Conspectus.” ‘“ De solido vivo, seu genere 
nervoso,” as if there were no living property in any of the animal solids but what 
is given to them by the nervous system; or, by the explicit declaration of CULLEN. 
that he considered the vital principle as “lodged in the nervous system.” 
The progress of the science has, I think, distinctly shewn that these ideas, as 
to the parts of the animal economy in which the peculiar laws of vitality operate, 
were limited and erroneous; although physiologists (trained in the schools of medi- 
cine where the authority of these and other teachers, adopting similar doctrines. 
has been held in just veneration) have been generally reluctant to admit the error. 
I have endeavoured, in papers laid at different times before this Society, to 
