Reductio ad Absurdum

If the consequences of a claim necessarily lead on to an absurd conclusion, then that statement is not likely to be credible.

This is a very old form of reasoning, thought to go back to the ancient Greeks and maybe before. It entails considering the consequences of a statement and if they are self-evidently non-credible then the statement is also likely to be non-credible. Moreover it can be used to validate a statement: if the reverse of the statement leads to a non-credible conclusion then the original statement is more likely to be credible.

Consider the statement: There is a very large number that is the largest number possible.

Obviously if you add 1 to any number you will get a larger number. So the statement that there is a largest possible number is non-credible, because we have considered the consequences of adding one to that large number and arrived a larger number, an absurd position if the original number was the largest possible number.

Or consider the statement: Phlogiston is a material, present on earth, that has negative weight. (See link for more on the history of phlogiston)

If there were a substance with negative weight this means it has a repulsion to the earth’s gravity not an attraction. Therefore it would float into space, and over time any Phlogiston on earth will evaporate into space and there will be none present near massive bodies such as the earth.

By considering the consequences of a statement then one can determine if that statement is likely to be false. This does not make the reverse of the statement true but it does lend some credibility to that reverse statement.


For more information:
http://en.wikipedia.org/wiki/Phlogiston_theory
http://en.wikipedia.org/wiki/Reductio_ad_absurdum

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