Informal Logic

One of the two main kinds of logic (the other is formal logic). Almost all the fallacies discussed in Lean Logic are informal. Informal logic considers the context, content and delivery of an argument; it recognises the fallacies that can destroy dialogue, and teaches how to avoid, or use, them. It is often thought of as the disreputable relation of formal logic, for it has no solid set of rules from which conclusions can be deduced. It tends to focus on mistakes, and its discussion of logic in the context of subjects such as religion, culture, law and the environment—by books such as this one—seems to violate the principles of logical rigour, which ought not to be contaminated by reference to any particular practical applications.

One of the symptoms of all this is that there is no definitive list of fallacies: you can (within limits) invent them as you go along, and it is possible to turn fallacies to advantage—using them, for instance, to persuade the other person of a conclusion for the wrong reasons (though a person who is fluent in informal logic should be able to expose such schemes). In fact, informal logic is altogether too informal for some tastes, and some logicians conclude with regret that it does not exist. Here, for example, is the logician Jaakko Hintikka, more in sorrow than in anger:

The [logical] structures I am concerned with uncovering are intended to be of the same kind as the structures studied in mathematical logic and foundations of mathematics. I have a great deal of sympathy with the intentions of those philosophers who speak of “informal logic”, but I don’t think any clarity is gained by using the term “logic” for what they are doing.I43


Old Gent (to Logician): Beautiful thing, logic.
Logician (to Old Gent): Provided it’s not abused.

Eugène Ionesco, Rhinoceros, 1959.I44


Related entries:

Fallacies, Fuzzy Logic, Lumpy Logic, Manners.

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David Fleming
Dr David Fleming (2 January 1940 – 29 November 2010) was an economist, historian and writer, based in London. He was among the first to reveal the possibility of peak oil's approach and invented the influential TEQs scheme, designed to address this and climate change. He was also a significant figure in the development of the UK Green Party, the Transition Towns movement and the New Economics Foundation, as well as a Chairman of the Soil Association. His wide-ranging independent analysis culminated in two critically acclaimed books, Lean Logic and Surviving the Future. A film about his perspective and legacy - The Sequel: What Will Follow Our Troubled Civilisation? - was released in 2019, directed by BAFTA-winning director Peter Armstrong. For more information, including on Lean Logic, click the little globe below!

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