SCIENCE.] INTRODUCTORY. 7 



a motive ? It is plain that there is no end to the 

 questions, one arising out of the other, that might be 

 asked in this fashion. 



Thus we believe that everything is the effect of 

 something which preceded it as its cause, and that 

 this cause is the effect of something else, and so on, 

 through a chain of causes and effects which goes back 

 as far as we choose to follow it. Anything is said to 

 be explained as soon as we have discovered its cause, 

 or the reason why it exists ; the explanation is fuller, 

 if we can find out the cause of that cause ; and the 

 further we can trace the chain of causes and effects, 

 the more satisfactory is the explanation. But no 

 explanation of anything can be complete, because 

 human knowledge, at its best, goes but a very little 

 way back towards the beginning of things. 



4. Properties and Powers. 



When a thing is found always to cause a particular 

 effect, we call that effect sometimes a property, 

 sometimes a power of the thing. Thus the odour 

 of onions is said to be a property of onions, because 

 onions always cause that particular sensation of 

 smell to arise, when they are brought near the 

 nose ; lead is said to have the property of heaviness, 

 because it always causes us to have the feeling of 

 weight when we handle it j a stream is said to have 

 the power to turn a waterwheel, because it causes the 

 waterwheel to turn ; and a venomous snake is said to 

 have the power to kill a man, because its bite may 

 cause a man to die. Properties and powers, then, arc 

 certain effects caused by the things which are said to 

 possess them. 



