Necessary and Sufficient

A crucial distinction between two kinds of condition required for your argument to hold true, or for an outcome or event to take place.

As the words make plain, a condition is necessary if the event could not have happened without it; it is sufficient if it is enough on its own to cause or trigger the event (although, in some cases, something else could just as well have caused it). Example: Before having the energy to play in tonight’s concert I need . . .

a. some bangers and mash [a sufficient condition, but steak and kidney pie would do just as well];

b. some supper [a necessary and sufficient condition].

But note that both “necessary” and “sufficient” are particular to the context: the context here is hunger, but of course I need to be a musician, too.

 

Related entries:

Formal Logic, Necessity.

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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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