A fallacy of generalization based on an improper transfer of properties of the whole to a part or from a part to the whole.
The classic example of this is the transfer of properties for which the body contributes as a whole to the body part of the central nervous system alone (the following is a gross simplification of the relevant discussion; those who wish to know more will find further information at the end of this article):
The brain decides on our behavior.
While the brain obviously makes a significant contribution to human behavior, it should not be ignored that functions and properties of other parts of the body (e.g., via hormones, sensory stimuli, hunger sensations, …) also influence the way we behave. The mistake is precisely to ignore this.
The term “mereological” is derived from the ancient Greek “méros” [μέρος], meaning “part” or “share”.
The mereological fallacy is closely related to the logical fallacies listed here under “fallacies of emergence” as well as the ecological fallacy from statistics, insofar as all of these involve the improper transfer of properties between the whole and its parts.
The difference is that this fallacy involves a misattribution of properties of a specific component within a system with those of the system as a whole, whereas the fallacies of emergence do not require this systemic aspect. The ecological fallacy, on the other hand, is specifically about statistical data which is interpreted at a wrong level of aggregation.
Therefore, the mereological fallacy can be seen as the equivalent of the rhetorical figures known as “pars pro toto” or “totum pro parte” (e.g., “head” or “soul” as a paraphrase for a person). However, while the rhetorical figure is normally understood as a mere figure of speech in which the inadmissibility of the reduction is clearly evident, this becomes a fallacy when this abstraction is assumed to be true and is not questioned.
A very good example of such a misconception (and also an alternative name for this) is the conceptualization of the visual apparatus as a kind of “cinema” in which the external world is projected onto the retina, from where it is viewed by an observer inside the brain (hence the name “homunculus”; Latin for “little human”).
Not only does this raise the obvious question of how this “observer” in turn perceives the image (possibly through an even smaller “human”?), but it attempts to explain the actual phenomenon through itself: instead of the human, a “human” now sees in the brain of the human, but the phenomenon of “seeing” is not explained thereby (circular reasoning).
Not only does this raise the obvious question of how this “internal observer” in turn perceives the image (possibly through an “even smaller human”?), but it attempts to explain the actual phenomenon through itself: instead of the observer, an “observer within the observer” now perceives the outside world, but the phenomenon of “seeing” is still not explained (circular reasoning).
This article is still incomplete. More examples will follow.