A fallacy of composition is a type of fallacy or logical failing in which one item is used to describe a larger class of things in which that one item is a part. There are essentially two ways in which this fallacy can occur. One way is when the aspects of one thing are considered to apply equally to a group in which that one thing belongs. Another way is when some aspect of one part of a larger thing is expanded to describe that entire thing.
The basic concept behind a fallacy of composition is in the unsupported extension of ideas or aspects of one thing to a larger population or object in which that thing is a part. This type of extension is not always fallacious, however, since there are instances in which something true of one item in a group or one part of an object is true of that entire group or object. The fallacy occurs when the extension is made without information that supports the idea that the extension is valid.
One of the simplest ways in which a fallacy of composition can occur is when one person or thing that is part of a group is used to represent the entire group. This may be thought of as over-generalization for a particular group, though it may occur more subtly than that. For example, someone can see that one wealthy person is male, therefore all wealthy people must be male, which is fallacious. This type of fallacy of composition can refer to many different aspects of people and groups, though it is not always false. For example, one full-size jet costs more to build than a wooden toy jet, and it is true to state that all full-size jets cost more to build than wooden toy jets.
The other common form of a fallacy of composition occurs when an aspect of the part of something is used to describe the entire thing. An example of this fallacy would be that the cells that make up a person are invisible to the human eye, therefore the entire person must be invisible to the human eye. This is, of course, fallacious, as the invisibility of the cells does not expand to the person or thing that is made up of those cells. Much like the other type of fallacy of composition, however, there are instances in which this is true, such as the fact that all of the bricks of a wall are solid matter, therefore the wall is solid matter.