Examples.  Where examples are used in argumentation they must serve as more than mere illustrations.  In exposition an illustration frequently explains, but that same example would have no value in argument because while it illustrates it does not prove.  A suppositious example may serve in explanation; only a fact will serve as proof.  The more inevitable its application, the more clinching its effect, the better its argumentative value.  Notice how the two examples given below prove that the heirs of a literary man might be the very worst persons to own the copyrights of his writings since as owners they might suppress books which the world of readers should be able to secure easily.  While these examples illustrate, do they not also prove?

I remember Richardson’s grandson well; he was a clergyman in the city of London; he was a most upright and excellent man; but he had conceived a strong prejudice against works of fiction.  He thought all novel-reading not only frivolous but sinful.  He said—­this I state on the authority of one of his clerical brethren who is now a bishop—­he said that he had never thought it right to read one of his grandfather’s books.

I will give another instance.  One of the most instructive, interesting, and delightful books in our language is Boswell’s Life of Johnson.  Now it is well known that Boswell’s eldest son considered this book, considered the whole relation of Boswell to Johnson, as a blot in the escutcheon of the family.

     THOMAS BABINGTON MACAULAY:  Copyright, 1841

Analogy.  In argument by analogy the speaker attempts to prove that because certain things are known to be true in something that can be observed they are likely to be true in something else which in so far as it can be observed is quite like the first.  We continually argue by analogy in daily life.  Lincoln was really using analogy when he replied to the urging to change his army leaders during the Civil War, that he didn’t think it wise to “swap horses while crossing a stream.”  Scientists use this method to draw conclusions when it is impossible to secure from actual observation or experiment a certain last step in the reasoning.  The planet Mars and the earth are similar in practically all observable matters; they are about the same distance from the sun, they have the same surface conditions.  The earth has living creatures upon it.  Hence—­so goes the reasoning of analogy—­Mars is probably inhabited.  Reasoning by analogy is used to prove that universal suffrage is good for the United States because it has been good for one particular state.  A student may argue by analogy that the elective system should be introduced into all high schools, because it has been followed in colleges.  It may be asserted that a leading bank president will make a good university president, because he has managed one complex institution.  The essence of all good reasoning by analogy is that the two things considered must be so nearly alike in all that is known that the presumption of belief is that they must also be alike in the one point the arguer is trying to establish.  This is the test he must apply to his own analogy arguments.