A dichotomy is a split into two parts that are considered to be either contradictory or mutually exclusive. The colors black and white are a classic example: either something is black, or it is white, with no room for overlap or alternatives. These divisions are used in a number of ways and in an assortment of fields, from philosophy to biology, and learning to think about them can be important. It is also a good idea for people to learn to identify a false dichotomy — on that is not, in other words, truly mutually exclusive.
The word is derived from the Greek dichotomia, which means “splitting in two.” Humans have obviously been using dichotomies for centuries, as they can be valuable tools for quickly identifying things and thinking about the world, although the danger of them is that they can quickly lead to oversimplification. This issue has been a common topic of discussion in many fields for thousands of years.
In biology, dichotomies are often used in keys, tools that are used to help people identify things. For example, a plant key might help people identify plants with a series of questions like “are the stems green?” Such a key is known as a “dichotomous key,” and these tools can be quite reliable when well designed, as they neatly rule out options until the user is left with one solid identification. There are many other applications for these divisions in fields like engineering, astronomy, economics, and so forth, and in some fields the word has a special meaning.
A well known example of a false dichotomy is the saying “you're either with us or against us.” In this case, the saying leaves out a third option, neutrality, setting up an “us vs. them” mentality that can be very dangerous. Such flaws in logic are often used in arguments, in the hopes of brow beating an opponent into conceding a point by forcing the issue. As a general rule, something is a false dichotomy when its elements are not mutually exclusive or contradictory, or when some other option or concept is left out.
Learning to identify these rhetorical tricks in politics can be extremely useful. For example, a politician running for office might say “we need better roads, therefore we need to raise taxes.” This of course leaves out the option of re-allocating existing funds, or of using funds in a more efficient way. The gross-over simplification of the issues is a trait common to many politicians, who want to encourage people to vote for them, along with their policies.