Ambiguous middle [term]

When the middle term in a syllogism is used with two different meanings (ex­tension), no valid conclusion can be drawn. This is an example of a “four-term fallacy”.

For example Open in Syllogism-Finder App:

Nothing light can ever be dark.
All feathers are light.
Therefore: no feather can be dark.

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 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 four-term fallacy.

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 Planet Venus), but differ in their intension, which can lead to intensional fallacies (example: “The evening star can be seen in the morning.”)

• Ambiguous Middle Term on Fallacy Files
• Four-term fallacy on RationalWiki