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The Merriam-Webster dictionary defines what it means to plagiarize:
"to steal and pass off (the ideas or words of another) as one’s own: use (another’s production) without crediting the source,
to commit literary theft: present as new and original an idea or product derived from an existing source."
But what happens when you honestly do have an original (to you) idea that you didn't find online, or hear from another? Case in point, a five-year old fishing with his grandma (who didn't really know anything about fishing at all) noticed, astutely, that fish liked to congregate under rocks and in the weeds along the bank. No one told him that. He just noticed that was where he tended to catch the most fish. If he were to write about his experience, should he cite someone for that information? Was it an original thought he could claim as his own, even though thousands of fishing experts before him had written about it? Is that common knowledge or easily accessible fact? Maybe it's all of the above. See how slippery the slope gets?
To determine if something is common knowledge many universities [including the Writing Center at UNC https://writingcenter.unc.edu/tips-and-tools/plagiarism/ ] use two criteria for student papers.
When I was a college freshman I wrote a paper on the comparisons between the "Rose of Sharon" in the Bible (Song of Solomon) and John Steinbeck's' Grapes of Wrath, and Rose of Sharon Joad - a character in the book. I was in college before computers (1974) and my comparisons were unique and a topic decidedly un-freshman like. The writing was excellent, far above par for a freshman, and the topic bizarre for an 18-year-old. So, my professor gave me an F on the paper, claiming I'd plagiarized it. I took it to the dean, horrified at the accusation. He happened to have been an expert on the subject - go figure, and after talking to me about the concepts in the paper, he pronounced it entirely original thought and an A paper. I got my A, got a B in the class, and moved on, angry at the professor forever for having doubted me. I've made it a point to cite references ever since, and still find that the rules aren't etched in concrete and opinions about what to cite often change from editor to editor.
In order to avoid plagiarising the work of other people you should cite or reference anything that is not common knowledge or based on an easily accessible fact. If in doubt, cite it and let your teacher, ghostwriter, editor, or publisher sort it out. Better to overcite, than under cite. That brings me to the reason for this post. Plagiarism a much larger problem than just a few sentences or paragraphs:
Plagiarism of Entire Books:
Over the past year I've noticed a disturbing trend with new authors. They come to me with a "book" and ask for several extra chapters to be written, or rewritten in order to complete it. I assume they have created the content they're giving me. I've been wrong, several times now — usually after running the content through a plagiarism checker [Turnitin.com, Grammarly, or https://www.duplichecker.com/ So, I just instituted a new policy, I must run everything a new client calls their content through a plagiarism checker before quoting them a price on their job. Why? Because three clients over the past year or so have plagiarized entire books. Was it deliberate? I don't know. I find it hard to believe that they were entirely clueless, as much as they protest they were unaware of what they were doing.
What they were doing was failing to tell me when they hired me is that their "book" was not really their book (as in their original thoughts), but that it was a collection of copied and pasted information they found on the Internet. In these books they have generally failed to cite anything or point to references for what they are talking about. They just found something they liked, and copied and pasted it into a document to create their "book." Or they hired third world, English as a second or third language writers to create a report (which was also mostly plagiarised from English websites) and used that.
Some people honestly don't understand what plagiarism is. Some don't care. Some don't realize it's a serious problem. I've experienced the whole topic of, "what is plagiarism and what is not," to be difficult even for "experts" - such as editors, ghostwriters, and publishers. As Corina Koch MacLeod and Carla Douglas wrote in an 2015 article on BookDesigners.com, "Plagiarism. You know it when you see it, but it’s as slippery as a Canadian sidewalk in winter when you try to define or explain it."
The lines between what is "common knowledge," or "easily accessible facts," as in information or facts most people know, like "bears hibernate" or Tennessee is one of the 50 states in the United States, versus information that needs to be cited, like "Chevy Trucks are better than Dodge Ram trucks" can be difficult to determine.
For instance, is the fact that "vinegar can kill 99.9% of germs" common knowledge, easily accessible fact, or is it something that should be cited? If you're a woman (not to be sexist here), that's pretty much common knowledge. I don't know many men who clean, so it's not such a common fact with them. In fact, a male editor for a housekeeping magazine (whom I would have assumed would be familiar with vinegar's cleaning properties, told me vinegar's natural germ killing properties were not common knowledge. He was upset and said that statement should have been cited. Really? So you can see the dilemma even professional writers have with what is plagiarism and what isn't. Apparently, even with the excess of information and easy access to information on the Internet today, the bar for common knowledge is what a five-year-old would be aware of — as in "fish swim in water," or "cats purr." Better to be too conscientious than not.
If the person writing the paper/book etc. is an expert in their field and they are writing about a topic like, oh let's say horses, then they can write about their personal experience from their professional expertise, but, even if you've known or believed something for years and know it to be fact, you must still cite it if it's not common knowledge, if it's an opinion, if it could be challenged.
When Should You Cite Your Sources?
The best practice for deciding whether to use a citation is to overcite, and let someone with a higher pay-grade (your ghostwriter, editor, publisher, etc.) make that decision.
When you're working with a ghostwriter, tell them if the writing you're giving them is original (your own), or borrowed, paraphrased, or rewritten from sources you found online, etc..
Consistency Matters More Than Talent
You don't have to be talented to succeed. You do need to be consistent.