EXAMPLES OF PRACTICAL REASONING 211 



spring and a train of intervening wheel-work. But Book in 

 if any one were explaining the causes of a chrono- 

 meter's exceptional accuracy he would never think 

 of mentioning these last at all. He would not dwell 

 on the properties of the coil of elastic steel, or on the 

 interaction of the ordinary toothed wheels, or on the 

 steel axes which make their interaction possible. 

 And why would he omit these causes ? He would 

 omit them because they would be assumed, because 

 there would be no discussion about them, and 

 because they are implied in the existence of all 

 watches and chronometers equally. If, however, 

 the case were reversed if all escapements and all 

 balances were alike, and there was no room for 

 superiority except in the main -spring and the 

 wheel-work the latter would be dwelt on, and 

 the former would be passed over, in any discussion 

 that turned on the causes of accurate time- 

 keeping. 



Let us take one case more. A man is hanging by or the causes 

 a rope which is fastened to a spike of rock, and is man who is 

 looking for samphire or birds' eggs on the face of a a a rope. g 

 sheer cliff. It is suddenly perceived by some of his 

 friends on the summit that the rope is frayed a 

 yard or two above his head. They are anxious for 

 his safety ; and if any one asked them why, they 

 would answer, Because his life depends on the rope 

 not breaking. Let us suppose, however, that the 

 rope is perfectly strong, but that the spike of rock 

 it is attached to shows signs of being about to 

 fall. The man's friends in that case will explain 



