156 GENERAL VIEW AND BASIS OF SCIENTIFIC RESEARCH. 



selves into the old ones, or at least require only some easy 

 modification of the hypothesis first assumed : the system 

 becomes more coherent as it is further extended. The 

 elements which we require for explaining a new class of 

 facts are already contained in our system. Different 

 members of the theory run together, and we have thus a 

 constant convergence towards unity. In false theories, the 

 contrary is the case. The new suppositions are something 

 altogether additional ; not suggested by the original 

 scheme; perhaps difficult to reconcile with it. Every 

 such addition adds to the complexity of the hypothetical 

 system, which at last becomes unmanageable, and is com- 

 pelled to surrender its place to some simpler explanation.' 

 ' The doctrine of phlogiston brought together many facts 

 in a very plausible manner combustion, acidification, 

 and others and very naturally prevailed for a while. 

 But the balance came to be used in chemical operations, 

 and the facts of weight as well as of combination were to 

 be accounted for. On the phlogistic theory, it appeared 

 that this could not be done without a new supposition, 

 and that a very strange one ; that phlogiston was an 

 element not only not heavy, but absolutely light, so that 

 it diminished the weight of the compounds into which it 

 entered. Some chemists for a time adopted this extrava- 

 gant view ; but the wiser of them saw, in the necessity of 

 such a supposition to the defence of the theory, an evidence 

 that the hypothesis of an element phlogiston was erroneous. 

 And the opposite hypothesis, which taught that oxygen 

 was subtracted and not phlogiston added, was accepted 

 because it required no such novel and inadmissible 

 assumption.' l 



With regard to the question, What is the mental faculty 

 by means of which we detect and apprehend truth ? all 



1 Whewell, Philosophy of the Inductive Sciences, vol. ii. p. 233-5. 



