The fallacy of Composition is committed when a conclusion is drawn about a whole based on the features of its constituents when, in fact, no justification provided for the inference. There are actually two types of this fallacy, both of which are known by the same name (because of the high degree of similarity).
The first type of fallacy of Composition arises when a person reasons from the characteristics of individual members of a class or group to a conclusion regarding the characteristics of the entire class or group (taken as a whole). More formally, the "reasoning" would look something like this.
Individual F things have characteristics A, B, C, etc.
Therefore, the (whole) class of F things has characteristics A, B, C, etc.
This line of reasoning is fallacious because the mere fact that individuals have certain characteristics does not, in itself, guarantee that the class (taken as a whole) has those characteristics.
It is important to note that drawing an inference about the characteristics of a class based on the characteristics of its individual members is not always fallacious. In some cases, sufficient justification can be provided to warrant the conclusion. For example, it is true that an individual rich person has more wealth than an individual poor person. In some nations (such as the US) it is true that the class of wealthy people has more wealth as a whole than does the class of poor people. In this case, the evidence used would warrant the inference and the fallacy of Composition would not be committed.
The second type of fallacy of Composition is committed when it is concluded that what is true of the parts of a whole must be true of the whole without there being adequate justification for the claim. More formally, the line of "reasoning" would be as follows:
The parts of the whole X have characteristics A, B, C, etc.
Therefore the whole X must have characteristics A, B, C.
That this sort of reasoning is fallacious because it cannot be inferred that simply because the parts of a complex whole have (or lack) certain properties that the whole that they are parts of has those properties. This is especially clear in math: The numbers 1 and 3 are both odd. 1 and 3 are parts of 4. Therefore, the number 4 is odd.
It must be noted that reasoning from the properties of the parts to the properties of the whole is not always fallacious. If there is justification for the inference from parts to whole, then the reasoning is not fallacious. For example, if every part of the human body is made of matter, then it would not be an error in reasoning to conclude that the whole human body is made of matter. Similiarly, if every part of a structure is made of brick, there is no fallacy comitted when one concludes that the whole structure is made of brick.
- A main battle tank uses more fuel than a car. Therefore, the main battle tanks use up more of the available fuel in the world than do all the cars.
- A tiger eats more food than a human being. Therefore, tigers, as a group, eat more food than do all the humans on the earth.
- Atoms are colorless. Cats are made of atoms, so cats are colorless.
- "Every player on the team is a superstar and a great player, so the team is a great team." This is fallacious since the superstars might not be able to play together very well and hence they could be a lousy team.
- "Each part of the show, from the special effects to the acting is a masterpiece. So, the whole show is a masterpiece." This is fallacious since a show could have great acting, great special effects and such, yet still fail to "come together" to make a masterpiece.
- "Come on, you like beef, potatoes, and green beens, so you will like this beef, potato, and green been casserole." This is fallacious for the same reason that the following is fallacious: "You like eggs, icecream, pizza, cake, fish, jello, chicken, taco sauce, soda, oranges, milk, egg rolls, and yogurt so you must like this yummy dish made out of all of them."
- Sodium and Chloride are both dangerous to humans. Therefore any combination of sodium and chloride will be dangerous to humans.