THE ART OF SCIENTIFIC INVESTIGATION 



premisses are seldom precise or unconditionally true. Especially 

 in biology premisses are only true under certain circumstances. 

 For careful reasoning and clarity of thought one should first 

 define the terms one uses but in biology exact definitions are often 

 difficult or impossible to arrive at. Take, for example, the 

 statement " influenza is caused by a virus." Influenza w^as 

 originally a clinical concept, that is to say, a disease defined on 

 clinical characters. We now know that diseases caused by several 

 different microbes have been embraced by what the clinician 

 regards as influenza. The virus worker would now prefer to 

 define influenza as a disease caused by a virus with certain 

 characters. But this only passes on the difficulty to the defining 

 of an influenza virus which in turn escapes precise definition. 



These difficulties are to some extent resolved if we accept the 

 principle that in all our reasoning we can deal only in probabili- 

 ties. Indeed much of our reasoning in biology is more aptly 

 termed speculation. 



I have mentioned some limitations inherent in the application 

 of logical processes in science; another common source of error 

 is incorrect reasoning, such as committing some logical fallacy. 

 It is a delusion that the use of reason is easy and needs no training 

 or special caution. In the following section I have tried to outline 

 some general precautions which it may be helpful to keep in mind 

 in using reason in research. 



Some safeguards in use of reason in research 



The first consideration is to examine the basis from which we 

 start reasoning. This involves arriving at as clear an understanding 

 as possible of what we mean by the terms we employ, and examin- 

 ing our premisses. Some of the premisses may be well-established 

 facts or laws, while others may be purely suppositions. It is often 

 necessary to admit provisionally some assumptions that are not 

 well established, in which case one needs to be careful not to 

 forget that they are only suppositions. Michael Faraday warned 

 against the tendency of the mind " to rest on an assumption " and 

 when it appears to fit in with other knowledge to forget that it 

 has not been proved. It is generally agreed that unverified 

 assumptions should be kept down to the bare minimum and the 



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