Generally speaking, facts that are common knowledge (for example, the date that WWII ended) need not be referenced, while facts that are not considered common knowledge in one’s field must be cited. Similarly, a quote from any source, words or information, even if paraphrased, or any ideas not one’s own must be cited. For instance, while it is acceptable to copy several paragraphs of text from a book and place them in a paper, if the source of the text (the author’s name and title of the work) is not identified, even if the text is well known (for example, an excerpt from Lewis Carroll’s Jabberwocky), it is considered plagiarism.
Similarly, it is considered plagiarism to take someone’s idea and then present it as one’s own work. However, it is not plagiarism when two (or more) people independently come up with the same new ideas. This is commonly termed simultaneous inspiration, and comes about as the result of people exposed to the same source and interpreting it similarly. This commonly occurs in the sciences, for example Newton and Leibniz’ seemingly independent invention of calculus.
There is some difference of opinion over how much credit must be given in non-academic settings, such as when preparing a newspaper article or historical account. Generally, reference is made to original source material as much as possible, and writers avoid taking credit for others’ work. The use of facts in non-academic settings (e.g. journalism, speeches), rather than works of creative expression, does not usually constitute plagiarism. However, if those giving a speech (e.g. politicians) have power over the lives of others, then they usually have a moral duty to ensure their claims are seen to be based on reliable and tracable evidence.