Editing Causality (section)

=== Necessary and sufficient causes ===<!-- This section is linked from [[Causes of schizophrenia]] -->
{{hatnote|A similar concept occurs in logic, for this see [[Necessary and sufficient conditions]]}}

Causes may sometimes be distinguished into two types: necessary and sufficient.<ref>Epp, Susanna S.: "Discrete Mathematics with Applications, Third Edition", pp. 25–26. Brooks/Cole – Thomson Learning, 2004. {{ISBN|0-534-35945-0}}</ref> A third type of causation, which requires neither necessity nor sufficiency, but which contributes to the effect, is called a "contributory cause".

;Necessary causes: If ''x'' is a necessary cause of ''y'', then the presence of ''y'' necessarily implies the prior occurrence of ''x''. The presence of ''x'', however, does not imply that ''y'' will occur.<ref name="CR">{{Cite web
 | url = http://www.istarassessment.org/srdims/causal-reasoning-2/
 | title = Causal Reasoning
 | website = www.istarassessment.org
 | access-date = 2 March 2016
}}</ref>
;Sufficient causes: If ''x'' is a sufficient cause of ''y'', then the presence of ''x'' necessarily implies the subsequent occurrence of ''y''. However, another cause ''z'' may alternatively cause ''y''. Thus the presence of ''y'' does not imply the prior occurrence of ''x''.<ref name="CR"/>
;Contributory causes: For some specific effect, in a singular case, a factor that is a contributory cause is one among several co-occurrent causes. It is implicit that all of them are contributory. For the specific effect, in general, there is no implication that a contributory cause is necessary, though it may be so. In general, a factor that is a contributory cause is not sufficient, because it is by definition accompanied by other causes, which would not count as causes if it were sufficient. For the specific effect, a factor that is on some occasions a contributory cause might on some other occasions be sufficient, but on those other occasions it would not be merely contributory.<ref name="Riegelman">{{Cite journal
 | last1 = Riegelman | first1 = R.
 | title = Contributory cause: Unnecessary and insufficient
 | journal = Postgraduate Medicine
 | volume = 66
 | issue = 2
 | pages = 177–179
 | year = 1979
 | pmid = 450828
 | doi = 10.1080/00325481.1979.11715231
}}</ref>

[[J. L. Mackie]] argues that usual talk of "cause" in fact refers to '''INUS''' conditions ('''i'''nsufficient but '''n'''on-redundant parts of a condition which is itself '''u'''nnecessary but '''s'''ufficient for the occurrence of the effect).<ref>{{Cite book|url=https://books.google.com/books?id=h38IAQAAIAAJ|title=The Cement of the Universe: A Study of Causation|last=Mackie|first=John Leslie|date=1974|publisher=Clarendon Press|isbn=9780198244059|language=en}}</ref> An example is a short circuit as a cause for a house burning down. Consider the collection of events: the short circuit, the proximity of flammable material, and the absence of firefighters. Together these are unnecessary but sufficient to the house's burning down (since many other collections of events certainly could have led to the house burning down, for example shooting the house with a flamethrower in the presence of oxygen and so forth). Within this collection, the short circuit is an insufficient (since the short circuit by itself would not have caused the fire) but non-redundant (because the fire would not have happened without it, everything else being equal) part of a condition which is itself unnecessary but sufficient for the occurrence of the effect. So, the short circuit is an INUS condition for the occurrence of the house burning down.