+--Cause and effect are so closely united that
when an effect is observed we assume that there has been a cause, and we
direct our argument to proving what it is. An effect is so associated with
its cause that the existence of an effect is a sign of the existence of a
cause, and such an argument is called an _argument from sign_. Reasoning
from sign is very common in our daily life. The wild geese flying south
indicate the approach of cold weather. The baby's toys show that the baby
has been in the room. A man's hat found beside a rifled safe will convict
the man of the crime. A dog's track in the garden is proof that a dog has
been there.
If the effect observed is always associated with the same cause, the
argument is conclusive. If I observe as an effect that the river has
frozen over during the night, I have no doubt that it has been caused by a
lowering of the temperature.
If two or three possible causes exist, our argument becomes conclusive
only by considering them all and by showing that all but one did not
produce the observed effect. If the principal of a school knows that one
of three boys broke a window light, he may be able to prove which one did
it by finding out the two who did not. If a man is found shot to death,
the coroner's jury may prove that he was murdered by showing that he did
not commit suicide. If there are many possible causes, the method of
elimination becomes too tedious and must be abandoned.
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