Informal Logic

© 2011 By Paul Herrick

A Survey of the Main Branches of Logic

It is common to distinguish “formal” and “informal” logic. Formal logic comprises those areas of logical theory that study the forms of reasoning independent of the content of reasoning. Formal logic is thus by its nature quite abstract, for it abstracts the form from the content. Sometimes formal logic is called “symbolic logic” because (thanks to Frege) it uses special symbols and formulas, similar to those used in mathematics, to represent the forms of reasoning. Since logical forms are abstract, they are well suited to symbolic expression.

Informal logic is concerned with the nonformal aspects of logical theory. The three most important branches of “informal logic” are:

  1. The study of definitions.
  2. The study of the informal fallacies.
  3. The study of inductive reasoning.

Let’s look briefly at each. A definition is an “explanation of the meaning of a word or phrase.” As we have seen, if we do not define the words we use, our reasoning may easily become sidetracked. The art of defining words is an important part of logical theory. I have provided some helpful sources in the Useful Links page.

A fallacy is “an error in reasoning.” An informal fallacy is “an error in reasoning that involves the content of the reasoning and not merely the form.” People “commit” informal fallacies of reasoning all the time in everyday life. You can also see people making the errors called “informal fallacies” if you watch a typical political debate on TV.

Like most of the other areas of logical theory, this one goes back to Aristotle. For Aristotle was actually the first logician in history to publish a catalog of common informal fallacies and to analyze the phenomenon systematically. The ability to spot informal fallacies of reasoning, both in your own reasoning and in the reasoning of others, is an important logical skill, and the study of the informal fallacies is an important part of logical theory. A huge number of different informal fallacies have been cataloged since the days of Aristotle. I have provided some resources in the Useful Links page.

Logicians have identified and carefully analyzed at least 12 different types of inductive reasoning, each commonly used in everyday life, in the courtroom, in science, and in many other real contexts. Understanding the different types of induction is an extremely useful logical skill, one that can help you reason better in many different aspects of life.

For example, during the 19th century, the logician John Stuart Mill (1806-1873) formulated the principles of induction that are still used in the laboratory when scientists run tests aimed at finding the causes of things. Every health department follows these principles first formulated by Mill, commonly known as “Mills Rules,” when tracking down the cause of an outbreak of a disease.

The subject of induction is vast. This is a marvelous, and useful, branch of logic. Some interesting links will be found on the Useful Links page.

And Beyond…

This concludes our “cook’s tour” of the main branches of introductory logic. If you, the reader, would like more—in particular, if you are interested in seeing where the subject goes after the basic concepts have been nailed down, the “Frontiers of Logic” page below surveys many of the latest developments. During the 20th century there was an explosion of creativity and discovery by logicians in the Western world, taking the subject into theoretical realms undreamed of by Aristotle, the Stoics, and even Gottlob Frege, the founder of modern symbolic logic. The journey beyond the basics is a fascinating one. I have attempted to present a taste of all of this in the Frontiers of Logical Theory page.

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