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. 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.
Categories of Common Knowledge
Widely known facts
Widely known scientific and historical facts—such as the molecular structure of water (H2O), or that Ernest Manning was the eighth Premier of Alberta—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 Lap-Chee Tsui, a Canadian geneticist, led a team that identified the gene behind cystic fibrosis—are widely known to some groups of people (professional geneticists) but perhaps not to you. Nevertheless, you would not have to cite the fact about Tsui, since it is common knowledge in the sense that no particular individual discovered this information.
On the other hand, as soon as your discussion includes an individual's thought, research, or analysis, you do have to cite. For example, if you read Gregory Marchildon’s article on Ernest Manning and wrote a paper in which you repeated Marchildon's claim that Manning’s conservative ideals and religious impetus continue to have currency among those challenging universal healthcare, you would need to cite Marchildon as the source of this idea.
Ideas or interpretations are usually not considered common knowledge, unless they are very widely held.
Ideas or interpretations are usually not considered common knowledge, unless they are very widely held. 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 Justin Trudeau’s swearing-in 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 Prime Minister Trudeau’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 Kennedy’s 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.
Adapted from Harvard College Writing Program (2019) https://usingsources.fas.harvard.edu/exception-common-knowledge
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