====== Ambiguous middle [term] ======

When the //middle term// in a [[glossary:syllogism|syllogism]] is used with two different meanings (<span maniculus>[[glossary:extension|ex­tension]]</span>), no valid conclusion can be drawn. This is an example of a "[[logic:formal_fallacies:four-term_fallacy:index|four-term fallacy]]". 

For example<span noprint> [[app>#celarent|Open in Syllogism-Finder App]]</span>:

> Nothing <u ambiguous "equivocation"><var>light</var></u> can ever be <var>dark</var>.
> All <var>feathers</var> are <u ambiguous "equivocation"><var>light</var></u>.
> <span conclusio>Therefore: <s invalid>no <var>feather</var> can be <var>dark</var>.</s></span>

===== Description =====

Obviously, the term "light", which in the above syllogism serves as the //middle term// that connects the two premises, is used to describe two different facts: firstly the presence of photons and secondly the absence of weight. Thus, it is an example of an [[glossary:equivocation:index|equivocation]], a misleading use of synonyms.

Since the identical terms do not describe the same thing, they are actually two different terms which only "coincidentally" look the same. Thus, this can be described as a variation of the [[logic:formal_fallacies:four-term_fallacy:index|four-term fallacy]].

==== Intensional ambiguity ====

But also the reverse case is possible: that two terms denote the same object (e.g. both, “morning star” and “evening star” refer to the [[wp>Venus|Planet Venus]]), but differ in their [[glossary:intension|intension]], which can lead to [[logic:emergence:ambiguity:intensional_fallacy|intensional fallacies]] (example: "<u questionable "illicit substitution of identicals">The //evening star// can be seen in the //morning//.</u>")

===== More information =====

  * [[http://www.fallacyfiles.org/ambimidd.html|Ambiguous Middle Term]] on //Fallacy Files//
  * [[https://rationalwiki.org/wiki/Four-term_fallacy|Four-term fallacy]] on //RationalWiki//