WHAT IS SCIENCE? 



A ; and, on the other hand, if A happens, it is sure to be 

 followed in due course by B. When we can discover 

 such a relation between two events, we say that A is the 

 cause of B and B the effect of A. A single example will 

 suffice for illustration. If my finger bleeds, it is because 

 I have cut it. The cutting, which necessarily precedes 

 the bleeding, is the cause ; the bleeding which necessarily 

 follows the cutting is the effect. 



However, this simple and familiar notion, like so many 

 equally simple and familiar to a first glance, appears 

 rather more complex and intricate on further examination. 

 The many difficulties which might be and have been 

 raised to the acceptance of this simple view are not 

 strictly relevant to our present purpose, but a few of 

 them may be noted for the information of the reader 

 unaccustomed to philosophical discussion. The first 

 difficulty is that there are certainly pairs of events, A 

 and B, one invariably preceding the other, which we' do 

 not regard as cause and effect ; for instance, birth invari- 

 ably precedes death, and yet we should not accept readily 

 the conclusion that birth is the cause of death. Again, 

 sometimes B, though always following A, also always 

 precedes another A ; day always follows night, but it 

 also always precedes night ; is day or night the cause, 

 or is there no relation of cause and effect involved ? 

 Once more, even when we are clear that there is a relation 

 of cause and effect involved, it is often difficult to say 

 precisely which, out of many alternations, is the cause, 

 Death, for instance, may be the effect of natural causes, 

 or of a hundred forms of accident or violence. We 

 know that it must always be the effect of one of them, 

 but we are so uncertain of which is the cause in each par- 

 ticular case that a special form of inquiry is thought 

 necessary. How is this uncertainty consistent with the 

 invariable sequence of effect after cause which seems 

 assumed by the use of those words ? Such difficulties 



