SCIENTIFIC METHOD 43 



now make fresh observations within a changed setting. 

 This last procedure constitutes a genuine experiment ; by 

 its means we are able to eliminate various antecedents, 

 and to adjudicate upon their causal relationship to the 

 observed phenomena. We thus approach more nearly to 

 the attainment of the causative sequence, and for this 

 reason experimental research is justly held as an invalu- 

 able technique ; it constitutes the most potent of all the 

 methods which are employed for the furtherance of 

 scientific knowledge. Indeed, it frequently happens that 

 no real advance is made as long as some definite procedure 

 of this type, the crucial experiment, is lacking. As an 

 illustration of this, I would refer you to a branch of 

 scientific knowledge which is not included in the special 

 lectures given at this meeting ; I allude to the causation 

 of disease by micro-organisms. It is not enough to 

 observe that any given disease is, under certain conditions, 

 communicable from one animal to another, and is there- 

 fore contagious ; it is not enough to observe that the 

 disease is associated with the presence and multiplication 

 of special forms of micro-organisms in the body tissues ; 

 the crucial experiment is still wanting. This consists, first, 

 in the isolation of the observed micro-organisms from all 

 others, and is attained by causing these, and these only, 

 to develop outside the body in appropriate media. It 

 consists, in the second place, in the introduction of such 

 isolated micro-organisms into the tissues of a healthy 

 susceptible animal. If, as the result of this introduction, 

 the disease is developed, then the crucial experiment is 

 decisive, the judgement is convinced, and the causative 

 sequence is so far determined. It was the genius of Koch 

 which, realizing the cogency of such experimental procedure 

 in the case of anthrax (wool-sorter's disease), first proved 

 that this disease was definitely due to the introduction 



