Occam's razor (also written as Ockham's razor, Latin lex parsimoniae) is a principle of parsimony, economy, or succinctness used in logic and problem-solving. It states that among competing hypotheses, the one that makes the fewest assumptions should be selected.
Suppose two explanations are equally likely. In this case the simpler one is usually better. Another way of saying it is that the more assumptions you have to make, the more unlikely an explanation is.
Example: Two trees have fallen down during a windy night. Think about these two possible explanations:
- The wind has blown them down.
- Two meteorites have each taken one tree down, and after that hit each other and removed any trace of themselves.
Even though both explanations are possible, several other unlikely things would also need to happen for the meteorites to have knocked the trees down (they would have to hit each other and also not leave any marks). In addition, meteorites are very rare. Since this second explanation needs several assumptions to all be true, it is probably the wrong answer. Occam's razor tells us that the wind blew the trees down, because that is the simplest answer and therefore probably the right one.
Occam's razor also often comes up in medicine when there are many explanations for symptoms and the simplest diagnosis usually is the correct one. If a child has a runny nose, they probably have the common cold instead of a rare birth defect. Medical students are often told, "When you hear hoof beats, think horses, not zebras".