+190.  Argument from Sign.+—­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.  If you find that your horse is lame, it would be difficult to prove which of the many possible causes actually operated to produce the lameness, though the attendant circumstances might point to some one cause and so lead you to assume that it was the one.

Under arguments from sign should be included also those cases when we pass directly from one effect to another that arises from the same cause; as, “I hear the windmill turning, it will be a good day to sail;” or, “These beans are thrifty, therefore if I plant potatoes here I shall get a good crop.”  In these sentences the wind and the fertile soil are not mentioned, but we pass directly from one effect to another.

As used by rhetoricians, arguments from sign include also arguments from attendant circumstances.  If we have observed that two events have happened near together in time, we accept the occurrence of one as a sign that the other will follow.  When we hear the factory whistle blow, we conclude that in a few minutes the workmen will pass our window on their way home.  Such a conclusion is based upon a belief established by an inductive process.  The degree of probability that it gives depends upon the number of times that it has been observed to act without failure.  If we have seen two boys frequently together, the presence of one is a sign of the probable presence of the other.  A camp fire would point to the recent presence of some one who kindled it.

In using an argument from sign care must be taken not to confuse the relation of cause and effect with that of contiguity in time or place.  Do not allege that which happened at the same time or near the same place as a cause.  If you do use an attendant circumstance, be sure that it adds something to the probability.