You can cite anything that you rely on to support an argument, counter an argument, build evidence for a conclusion you are making, for anything you represent, quote, summarize or paraphrase.
You'll want to cite whether what you cite is copyrighted or in the public domain.*
*Public domain is, for example, most US government documents, material published in the United States before 1925--this date goes up each year in January.)
The scholarly conversation is ongoing. It reaches back to the earliest printed sources and continues all around you. When we write a paper or publish a book or article, we are engaging in a public exchange of ideas. Each engagement is built on previous engagements, whether we are countering a previous assertion or building on it. We are required to acknowledge it. But, it is also to our benefit. It places us within that conversation.
