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A fallacy of division occurs when one reasons logically that something true of a thing must also be true of all or some of its parts.

An example:

  1. A Boeing 747 can fly unaided across the ocean.
  2. A Boeing 747 has jet engines.
  3. Therefore, one of its jet engines can fly unaided across the ocean.

The converse of this fallacy is called fallacy of composition, which arises when one fallaciously attributes a property of some part of a thing to the thing as a whole.

Another example:

  1. Functioning brains think.
  2. Functioning brains are nothing but the neurons that they are composed of.
  3. If functioning brains think, then the individual neurons in them think.
  4. Individual neurons do not think.
  5. Functioning brains do not think. (From 3 & 4)
  6. Functioning brains think and functioning brains do not think. (From 1 & 5)

Since the premises entail a contradiction (6), at least one of the premises must be false. We may diagnose the problem as located in premise 3, which quite plausibly commits the fallacy of division.

An application: Famously and controversially, in the philosophy of the Greek Anaxagoras (at least as it is discussed by the Roman Atomist Lucretius), it was assumed that the atoms constituting a substance must themselves have the salient observed properties of that substance: so atoms of water would be wet, atoms of iron would be hard, atoms of wool would be soft, etc. This doctrine is called homeomeria, and it plainly depends on the fallacy of division.

If a system as a whole has some property that none of its constituents has (or perhaps, it has it but not as a result of some constituent having that property), this is sometimes called an emergent property of the system.

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