330 THE SCIENCE OF LOGIC 



theory neglected to secure and examine an experimental instance, 

 because they assumed that if examined it would corroborate the 

 old one ; the Copernicans were not sufficiently scientific to 

 question this ; while the inferences of both parties alike reveal to 

 us how comparatively elementary was the scientific knowledge 

 that prevailed in those days about the laws of motion. 



It is important in the next place to note, and to guard 

 against, the natural tendency to neglect NEGATIVE instances. Strik 

 ing instances of a coexistence or sequence impress us, drawing 

 off our attention from the cases in which the coexistence or se 

 quence has not occurred ; and so we are tempted to see a causal 

 connexion in what may be merely casual. Remembering the 

 few instances in which our dreams have been &quot; fulfilled,&quot; and 

 ignoring the far more numerous instances in which they have 

 not, we are tempted to think that dreams are really prophetic. 

 It has been already pointed out (238) that non-observation, 

 whether of instances or of operative influences, does not prove 

 their non-existence. 



Non-observation of operative influences is one of the greatest 

 dangers to induction. The accuracy of our hypotheses and 

 generalizations depends upon the success with which we isolate 

 our phenomena, eliminating only what is unessential, and taking 

 into account all that is essential, to their occurrence. It was 

 commonly believed in the seventeenth century that a wound 

 could be healed by the application of a certain salve or powder 

 to the instrument that caused the wound, the latter being kept 

 clean and cool during the process of healing. The cure was 

 attributed to the &quot;sympathetic&quot; influence of the salve, rather 

 than to the unobserved recuperative forces of the patient s con 

 stitution when left to act in favourable conditions. As De 

 Morgan well remarks, 1 &quot;If we remember the dreadful notions 

 upon drugs which prevailed, both as to quantity and quality, we 

 shall readily see that any way of not dressing the wound would 

 have been useful &quot;. This form of fallacy is particularly prevalent 

 and difficult to avoid in exploring the causes of complex social, 

 economic, political, and religious phenomena. 



(2) MAL-OBSERVATION is the wrong interpretation of what 

 falls immediately under sense perception. It is due to the mis 

 leading interference of unconscious inference arising from habitual 

 beliefs, prejudices, and mental tendencies. Many people believe 



1 Budget of Paradoxes, p. 66 ; apud WELTON, op. cit., ii., p. 264. 



