14 THE CAUSATION OF DISEASE. 



of the molecules in the cell ; (c) finally, the relations of the cells 

 among themselves. 



It may be thought that nothing in this or the preceding- 

 chapters has any connection with the causation of disease ; 

 but I hope that the foregoing remarks have helped us in this 

 wise — that they have placed us in a position to define the 

 cause of life ; and it is very important that we should have 

 clear ideas as to the causation of life, because if we cannot 

 give an intelligible definition of life, we certainly cannot of 

 disease, for disease is a peculiar mode of life. 



How, then, shall we define life, and what shall we say is 

 the cause of it ? In order to answer this, let us take the 

 simplest form of life — a unicellular organism, or simple pro- 

 toplasmic cell. Such a simple cell consists of a peculiar 

 grouping of certain kinds of atoms into molecules, and a 

 peculiar grouping of the molecules among themselves, each 

 grouping — the atomic and molecular — being such that, when 

 the protoplasmic cell is placed within a certain environment 

 — when there is, that is to say, a certain material arrange- 

 ment round and about the cell, a certain interaction takes 

 place between the cell and the cell-environment. This inter- 

 action we term life;* it constitutes the life of the cell, and the 

 cause of life, according to our definition of cause, is the sum of 

 the material conditions of the cell and those material conditions 

 of cell-environment which share in the mutual interaction^ 



* By saying that life is a certain interaction of cell and cell-environment, I 

 am, of course, not attempting a logical definition of it. After life is over a 

 " certain interaction " occurs between the two. The above statement, although 

 a very inadequate definition of life, is correct so far as it goes, and is sufficient 

 for our purpose. 



f I have here assumed that the phenomena of life may be expressed in 

 terms of natural laws, for the peculiar interaction of cell and cell-environ- 

 ment, is in the last resort a chemical action, and chemistry, as we have seen, is 

 nothing else than molecular physics. In the language of Huxley, " Life is a 

 form or mode of ordinary force." This is now the prevailing view. A few 

 writers, however, still cling to the old notion that life is sustained by a force 

 which U <juitc unique, which cannot, namely, be correlated with ordinary 

 chemical and physical forces. Thus Beale says: "Life is a power, force, or 

 property of a special and peculiar kind, temporarily influencing matter and its 

 ordinary forces, but entirely different from, and in no way correlated with, anv 



