The Exception: Common Knowledge

The only source material that you can use in an essay without attribution is material that is considered common knowledge and is therefore not attributable to one source. Common knowledge is information generally known to an educated reader, such as widely known facts and dates, and, more rarely, ideas or language. Facts, ideas, and language that are distinct and unique products of a particular individual's work do not count as common knowledge and must always be cited. Figuring out whether something is common knowledge can be tricky, and it's always better to cite a source if you're not sure whether the information or idea is common knowledge. If you err on the side of caution, the worst outcome would be that an instructor would tell you that you didn't need to cite; if you don't cite, you could end up with a larger problem.

If you have encountered the information in multiple sources but still think you should cite it, cite the source you used that you think is most reliable, or the one that has shaped your thinking the most.

Categories of Common Knowledge

Widely known facts

Widely known scientific and historical facts—such as the molecular structure of water (H2O), or that Andrew Jackson was the seventh president of the United States—generally count as common knowledge. You can include such facts in your writing without citation and without fear of committing plagiarism. Other facts that count as common knowledge—for instance, that Franz Boas, the distinguished American ethnologist, held the first academic appointment in anthropology in the United States—are widely known to some groups of people (professional anthropologists) but perhaps not to you. Nevertheless, you would not have to cite the fact about Boas, since it is common knowledge in the sense that no particular individual discovered this information (say, through archival research at Columbia University, where Boas taught). On the other hand, as soon as your discussion becomes more specific and puts forth assertions that would be the product of an individual's thought, research, or analysis, you do have to cite. For example, if you read Anthony Abraham Jack's The Privileged Poor and then wrote about the conclusions he drew from his study of economically disadvantaged students at elite colleges, you would need to cite his work.

Ideas or interpretations are usually not considered common knowledge, unless they are very widely held.

Ideas or interpretations generally do not count as common knowledge. If you read in R.A.C. Parker's history of World War II that British Prime Minister Neville Chamberlain did not have to agree to the 1938 Munich Pact with Hitler, and that he could have chosen an alternate path, you would need to acknowledge the source, since this judgment is Parker's theory rather than a widely agreed upon fact. (Historians disagree on the factors that led Chamberlain to agree to sign on to the Munich Pact.) Some interpretations or opinions (rather than facts) have entered the realm of common knowledge and need not be cited. If you were to introduce the claim that culture provides a means by which humans adapt to their environments, you would not need to cite a source for this claim, since it is almost universally held by anthropologists. But, if you were unsure that this was the consensus view among anthropologists, you would be best served simply to cite the source. On the whole, opinions or interpretations do not enter the realm of common knowledge as easily as historical or scientific facts.

Verbatim language drawn from a source is rarely common knowledge, unless the formulation is widely known.

You must always provide a citation for quotations you use in your writing. The only—and rare—exceptions to this rule concern well-known quotations that have entered the realm of common knowledge. For example, if you were writing a paper about President Obama's Inaugural Address, you would need to cite your source for any quotations you used from the speech. However, if in the course of that paper you compared one of President Obama's lines to this very well-known phrase from John F. Kennedy's Inaugural Address, "Ask not what your country can do for you—ask what you can do for your country," you would not need to provide a citation for that one phrase. However, if you were to analyze Kennedy's speech substantively and quote additional lines, then you would need to cite anything you quoted from his speech so that your readers could confirm the original language of the speech. If you are not sure whether a quotation is common knowledge, cite it.