INUS condition
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.
INUS Condition Media
The sun being above the horizon is a necessary condition for direct sunlight; but it is not a sufficient condition, as something else may be casting a shadow, e.g., the moon in the case of an eclipse.
That a train runs on schedule is a sufficient condition for a traveller arriving on time (if one boards the train and it departs on time, then one will arrive on time); but it is not a necessary condition, since there are other ways to travel (if the train does not run to time, one could still arrive on time through other means of transport).
Being in the purple region is sufficient for being in A, but not necessary. Being in A is necessary for being in the purple region, but not sufficient. Being in A and being in B is necessary and sufficient for being in the purple region.
Related pages
References
- 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.