Here's why:
* Giving Credit: Even though the work isn't copyrighted, someone created it, and acknowledging their contribution is good scholarly practice. It shows respect for their work and avoids plagiarism, even if unintentional.
* Supporting Your Arguments: You're using the information to support your claims; properly identifying the source allows your reader to verify your information and understand the basis of your argument.
* Avoiding Ambiguity: If you directly quote or paraphrase a public domain work, indicating the source helps the reader differentiate your ideas from the original author's.
* Contextualizing Your Research: Mentioning the source helps to situate your work within a broader scholarly context.
How to acknowledge public domain sources:
* Footnotes or Endnotes: A simple phrase like "See [Name of Work], [Date]" is sufficient.
* In-text Citations (modified): Adapt your citation style to include the author and work's title, even without a date or page number if unavailable. For example, "(Shakespeare, *Hamlet*)"
* Bibliography/Works Cited: Include the public domain source in your bibliography. This is especially important if the source is less well-known or if the information is critical to your argument. Include as much information as you can find (author, title, date if known, publisher if known etc).
In short, while the legal requirements are different, ethical and academic best practices still necessitate acknowledging public domain sources used in your research. This ensures transparency and strengthens the credibility of your work.