INUS condition stands for an insufficient, but necessary part of an unnecessary but sufficient condition. John Mackie introduced the term in the 1960s.
Mackie uses the example of a house burning: There was an electric short circuit that caused the fire. There was flammable material nearby, which started to burn after the short circuit happened. He then tries to explain the statement: "The short circuit caused the fire", according to the INUS condition:
- The short circuit is not a sufficient part of the condition "short circuit and flammable material".
- The short circuit is a necessary part.
- The condition "short circuit and flammable material" is not necessary for the result (house burning). It can be replaced by other conditions (for example "lightning and flammable material", or "arson") to get the same result.
- "short circuit and flammable material" is a sufficient condition, as it will always result in the house burning.
INUS conditions need prior experience: Only after you have seen a house burning after a short-circuit putting flammable material on fire several times can you deduce that it is indeed sufficient.
- John Leslie Mackie: Causes and Conditions. ''American Philosophical Quarterly''. 1965, 2(4). pp. 245–264. http://www.jstor.org/stable/20009173.
- John Leslie Mackie: ''The Cement of the Universe: A Study of Causation''. London 1974: Oxford University Press. especially chapter 3: Causal Regularities, pp. 59–87.
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