In Ecclesiastes 1:9, Solomon laments:
What has been will be again,
What has been done will be done again;
There is nothing new under the sun.
To this the writer may add that what has been written will be written again, and there is no new text committed to paper. There are those who might contend, with some evidentiary support, that every story derives from Shakespeare, or the Greeks, or religious texts. Joseph Campbell's The Hero with a Thousand Faces even lays out the archetypal heroic quest, the pattern a hero can be expected to follow, and how this plays out within comparative mythology. One might find the theories reflected and confirmed in Star Wars and Harry Potter, in Hemingway and Fitzgerald, and indeed in just about any book, film, or story that does not seek actively to undermine them.
We might add that most romances follow some permutation of the same few plots, as do most mysteries, action adventures, etc. In academic writing, similarly, there are only so many forms an argument might take. Poetry has a little more creative room, but the practiced poet still learns classical forms, then applies them, avoids them, tweaks them--but in any event begins with poetic conventions in some manner.
What, then, is originality? Can a writer create something original? At the risk of promoting tautological thinking, the answer, as so often proves true, depends on the definition. The American Heritage Dictionary provides the following most relevant possibilities:
1. Preceding all others in time; first.
2.a. Not derived from something else; fresh and unusual . . . .
2.b. Showing a marked departure from previous practice; new . . . .
3. Productive of new things or new ideas; inventive . . . .
Taking these in order, it may or may not be possible to be original under definition 1. Any story one writes now cannot be the first story. The like is true for a sentence, an essay, a paragraph, or a blog post. One could coin a word, or a use for a word or phrase, but this would only be an instance of originality within a larger text. On the other hand, barring plagiarism or a remarkable coincidence, the words one uses, arranged in the way that person arranges them, will certainly be the first such arrangement.
The second definition is difficult. Anything I write is arguably derived from something else I have read, or seen, or heard. Does this render anything I write unoriginal? Or is a fresh and unusual presentation of a text derived from something else enough to overcome the first part of that definition?
We see again an issue with 2.b. and 3. What is "new"? Must we have a matter of jamais vu (something never seen before), or can something evoking deja vu (seen before) if in different context or presentation, or presque vu (almost seen before) suffice? And does my having read about and analyzed those French phrases in the context of Catch-22 render this entire paragraph unoriginal?
My solution to the endless circle of maybe presented here comes through how we think about writing, as something separate from storytelling or arguing. Writing is not simply telling a story or presenting an argument. Rather, every written text is a new construction. Writing consists of arranging words in such a way that it presents a story or argument to the reader or, ideally, readers. One could argue that every tragedy is derived from the Greeks, but that doesn't mean that Sophocles' prior work rendered Hamlet a mere derivation. A biography is necessarily derived from history (however recent)--but Sandburg's five-volume biography of Lincoln is still something different from that of Donald, in turn different from that of Oates or Goodwin. The writing is something different, fresh. How we put the story or argument into words makes it original.
Given this, given that every text written can be deemed original for being constructed differently, the conscientious writer should focus on how the words and sentences and paragraphs work, how they all fit together. You are creating something that will be the first, and last, of that text. Build carefully, and build proudly.