WHAT IS A DISEASE? 



By CHARLES MERCIER, M.D., F.R.C.P., F.R.C.S. 



A generation or two ago it used to be a favourite dodge of 

 counsel to begin their cross-examination of a witness who 

 claimed to be an expert in mental diseases, by asking him to 

 define insanity. The dodge was well calculated to confuse the 

 witness, and to take the conceit out of him ; besides giving 

 to a not too intelligent jury the impression that a person 

 who could not define his subject could not be an authority 

 upon it ; but if the witness had had the astuteness to turn 

 the tables, and ask his cross-examiner for a definition of law, 

 counsel would have been equally confounded. It is, in fact, 

 evident, and it is remarkable, that the fundamental concepts 

 in every calling are the most difficult to define, and that the 

 definition of them is not arrived at until late in the history 

 of that calling; and it is even more remarkable that the want 

 of valid definitions of fundamentals is for a long time but 

 little felt, and does not seriously interfere either with prac- 

 tice or with the advance of knowledge. 



If a committee consisting of the Lord Chancellor and all 

 the judges were appointed to draw up a definition of law 

 it is very unlikely that any two of them would agree ; but the 

 absence of a definition does not embarrass the administration 

 of the law. It would be as hard to find an engineer who 

 could frame a good definition of matter and force as to find 

 another engineer to agree with him ; but the want of such 

 a definition does not hinder engineers in building bridges or 

 devising machines. No soldier can define war, no parson can 

 define religion, no financier can define finance, nor any mer- 

 chant what is meant by commerce ; but yet soldiers can make 

 war, parsons can preach religion, financiers can raise loans, 

 and merchants can trade, without ever feeling the want of these 

 definitions. It is not surprising, therefore, that doctors have 

 formulated no definition of what is meant by a disease, nor 

 that the want of such a definition has not embarrassed the 



