A fallacy of division occurs when someone makes the argument that what is true of a whole object must be true of its constituent parts, without ample evidence to support this idea. One simple example of this fallacy is a situation in which someone argues that since the ocean when seen as a whole is blue in color, then each drop of water individually must also be blue in color. A fallacy of division occurs in this statement since water on its own is essentially colorless, and the properties of water droplets en masse and their collective reflectivity does not represent each droplet alone.
While fairly simple in nature, a fallacy of division can occur in two ways. One way in which this fallacy can happen is when someone views the properties of an entire thing and assumes the parts of it must also have such properties. This is seen in the previous example of the ocean and the individual drops of water. A fallacy of division can be avoided through more careful consideration of each element of an object and true statements such as “the ocean is reflective so individual drops of water must be reflective” can be made.
The other way in which someone can make a fallacy of division is through the assumption that the actions or beliefs of an entire population must represent the actions or opinions of each person in the population. Someone may, for example, consider a country that is quite wealthy and assume that each person within that country must also be wealthy. The reality of this type of situation, however, is much more complex than such an assumption provides for. This type of fallacy of division can be avoided through careful consideration of population samples prior to such statements being made.
In contrast to the fallacy of division is the fallacy of composition, in which a single item or individual is seen as representative of a much larger object or group. This fallacy can be committed by someone touching a droplet of water and seeing it disperse, then assuming that the ocean would disperse in much the same way when struck from a great height. The mass of water together in a large body like the ocean creates a surface with greater strength than a single droplet of water, and injuries often occur as a result of applying this type of fallacy. There are some statements and objects that can defy both a fallacy of division and composition, such as the argument that “bricks are solid, so the wall they make must be solid; and this wall is solid, so the bricks it is composed of must be solid.”