If you are familiar with the notion of “common knowledge” from earlier writing experiences, you may have noticed that its definition is easy to state, but can be hard to apply in a particular case. The “common” way to talk about common knowledge is to say that it is knowledge that most educated people know or can find out easily in an encyclopedia or dictionary. Thus, you might not know the date of the most recent meeting of the Federal Reserve, but you can find it out quite easily. Further, the term “common knowledge” carries the sense of “communal” knowledge—it is community information that no particular individual can fairly claim to own. One sign that something is community knowledge is that it is stated in 5 or more sources. So, if it’s known to educated people, or can be easily looked up, or appears in many sources, it is likely to be “common knowledge” and so does not need to be cited.
But here is where things become tricky: As you write papers in college and move deeper into your field of study, what counts as common knowledge becomes much less clear. Within a given discipline, there is a body of common knowledge that an outsider (even an educated college student who doesn’t happen to be in your field) might not know. For example, within psychology, it is common knowledge that chimpanzees recognize themselves in a mirror; in literature, it is common knowledge that James Joyce is a major modernist author. In referring to the mirror test or calling James Joyce a modernist, you wouldn’t need to cite anyone. But as soon as you begin to say something, for instance, about what the results of the mirror test mean for a model of consciousness, you would need to cite a source. The point, then, is to think about your audience: What has been said in the class or repeated in textbooks and other sources often enough to suggest that it is common knowledge within the discipline?
Because the notion of “common knowledge” is ambiguous and depends on context, you should always check with a professor or TF if you have any doubts. Some reference books will say “if in doubt, cite it,” but you don’t want to over-cite, so check with your readers to try to fix the line between common and specialized knowledge.
Sometimes you become so conversant in a subject that you can explain complex theories, methodolologies, or historical timelines without reference to a source. You may notice this phenomenon as you research and write your senior essay. At this point, you’re becoming an expert in the field and things may start to seem obvious to you that are not obvious to an intelligent lay reader. You will want to check with your department about the level of expertise you’re expected to assume. You may also want to show your writing to a Residential College Writing Tutor, a Writing Partner, or a friend who’s a good reader. As a senior essay writer, you will probably need to cite less than you used to, but more than you may think.
This advice about “common knowledge” is true for all disciplines—think about your audience and the course attitude, recognize when you’re writing as an expert, and always check with professors if you’re in doubt. The sciences, however, have a somewhat different notion of “common knowledge,” coming partly out of research practice and partly out of more collaborative work methods. Ideas, findings, and methodologies that are new knowledge (and therefore specialized rather than common knowledge) become old knowledge more quickly in the sciences. The answer, again, is to consider the messages you’re getting from the course about what concepts are common or foundational, and to check in with professors or TFs.