Fallacy of composition is a type of logical fallacy, meaning a flaw in reasoning that weakens an argument or a trick of thought used as a debate tactic. It occurs when the properties of a whole and its parts are mistakenly thought to be transferable from one to the other.
Furthermore, it’s also known as the “exception fallacy” and “faulty induction”, and it is the opposite of the fallacy of division.
Fallacy of composition arises when someone argues that something must be true of the whole because it is true of some parts of the whole.
It’s considered to be fallacious because the collective (the group as a whole) and distributive (individual members of a group) don’t necessarily need to have the same properties. As explained here:
One makes the mistake of attributing to a group (or a whole) some characteristic that is true only of its individual members (or its parts), and then makes inferences based on that mistake.W. H. Halverson, Concise Logic (1984).
As such, this fallacy takes the following logical form:
- Y is part of X.
- Y has property A.
- Therefore, X has property A.
An example would be:
- “Every part of this machine is light in weight, therefore this machine as a whole must be light too”.
This type of reasoning doesn’t hold water: even if the single parts of the machine are “light”, it doesn’t follow that their total combined weight must also be “light.” The arguer doesn’t take into account the fact that while the whole and its parts can possess the same attributes, it is not necessarily so.
- “If all the players in our team are the best players in their positions, then this must be the best team.”
- “If I leave home earlier in the morning, I can avoid the traffic jam. Therefore, if everyone leaves home earlier in the morning, everyone will avoid the traffic jam.”
- “Both sodium (Na) and chlorine (Cl) are harmful to humans. Therefore, table salt (NaCl) must also be harmful.”
- “The universe must be caused by something because everything in the universe has a cause.”
- “If one runner in a race runs faster, he or she can win. Therefore, if every runner in the race runs faster, they can all win.”
- “The legs of this chair are made of wood, so the whole chair must be made of wood.”
- “Trees are made of atoms, and atoms are not visible to the eye. Therefore, trees are not visible to the eye either.”
American Indians are disappearing. That man is an American Indian. Therefore that man is disappearing.
Because a bus uses more gasoline than an automobile, therefore all buses use more gasoline than all automobiles.I.M. Copi, Introduction to Logic, 3rd edition (1968).