330 THE BACTERIOPHAGE AND ITS BEHAVIOR 



which he is supporting, for another it favors his. This is precisely the 

 reason why indirect argument proves nothing. It neither supports nor 

 condemns, for by a clever argument each one can "make it fit" with the 

 hypothesis which he is defending. 



An indirect argument lacks even a tentative value, as is shown by 

 innumerable examples in the history of the sciences. In order to 

 demonstrate the validity of any hypothesis, or rather to transform an 

 hypothesis into an experimentally demonstrated certainty, it is necessary 

 to support this hypothesis by direct arguments, that is to say, by indis- 

 putable facts which automatically and effectively exclude any and all 

 other hypotheses. 



In reality, it is all reduced to a question of logic. It is necessary first 

 to give the question a logical expression, and then to ascertain whether in 

 the present state of our knowledge we are able to provide the answer to 

 this question. 



Is there a property characteristic of life which will allow us to deter- 

 mine whether a being is living or dead, depending upon whether the 

 being in question does or does not possess this property? If there is 

 such a characteristic property, does or does not the bacteriophage prop- 

 erty possess it? Briefly stated, this is the whole question. 



If the bacteriophage principle does not possess this property, nothing 

 can justify the statement that it is living. If it does possess it, discus- 

 sion must end, for a being which possesses the characteristic by means of 

 which we recognize life is necessarily living, and no amount of indirect 

 argument can transform a living being into an inert one. 



2. THE CRITERIA OF LIFE 



What are the criteria of life? Can we consider as criteria such attri- 

 butes as the faculty of motion, of increasing in volume, of exteriorizing 

 a force, of exerting serial actions? Certainly not. For beyond ques- 

 tion there are living beings which lack motihty; an inert crystal may 

 increase in volume; the wind gives expression to a physical force; tar- 

 nishing of tin transmits itseK serially; and yet none of these things are 

 caused by living beings. 



What is it, then, that tells us that a being is Hving? 



We do not know what life is, simply because as yet we are unable to 

 identify that physico-chemical property which confers on matter the 

 living state. But even if we do not yet know the essence of life we do 

 know some of the resultants of it. We know that it is a composite of 



