A thesis is a single sentence that expresses the controlling idea of a written work. A "controlling idea" is the core idea that drives a writing project's goals and structure.
Theses most often appear near the beginning of an essay. In short essays, the thesis is usually placed in the first paragraph, but this isn't always the case. It's common practice to position the core idea near the start of the essay, and then use the rest of the essay to explain it, and to provide the sub-claims and evidence that support it.
It's also common for a thesis to change during the writing process, most often during the research and drafting stages. This is normal, and it often indicates that the writer has done some critical thinking about his or her work. If a writer's thoughts and opinions don't change during the writing process, he or she may not have learned much (or anything) as a result of it. However, the eventual production of a solid draft of the thesis is important, because it anchors the the writing process and ensures that the project is on track.
What's the difference between a good thesis and one that needs more work? A good thesis presents a topic that makes readers want to keep reading. The best way to write a good thesis is to begin by making sure that your topic interests you. No matter the topic, it's likely that there's something about it — some thesis within it — that interests you and your potential readers.
A thesis must also be compelling. It must make readers want to understand more about your argument, either by involving them in it, or by making its claim in such a way as to encourage them to keep reading. A thesis, especially one that is presented at or near the beginning of an essay, should act like a road sign, signaling readers about what's ahead. Readers should not be surprised by the reasoning and evidence you use to support your essay's central claim.
A solid claim also differentiates a strong thesis statement from an inferior one. Your claim should be specific and clear. Even complex ideas can be stated as thesis statements when they reflect a strong grasp of the subject matter and efficient use of language.
A thesis can also be thought of as an answer to a question that's worth asking. A thesis question drives the composition of an argument. Open-ended questions are often most effective, because they help you to generate, revise, and focus your thesis as you write. Thesis questions can take a number of forms, as follows:
Questions of Fact. Common questions of fact include those that question the truth of a currently-held belief. Consider the following thesis statement, which is an answer to a question of fact:
The U.S. has long been recognized as the world leader in technical innovation, but recent evidence suggests that this may no longer be true.
Questions of Preference. These driving questions ask whether something is preferable to something else, as in the following thesis statement:
Science fiction is a more intellectually-worthwhile genre of writing than fantasy or adventure.
Questions of Definition. For example:
A two-by-four no longer measures two inches by four inches due to a change that occurred in the logging and lumber industries decades ago.
Questions of Interpretation. These are one of the most common types of thesis questions in academic writing. Questions of interpretation ask how something can be described or analyzed, as in this example:
Martin Luther King, Jr. argued for a positive peace — one that included justice — rather than a negative peace, which simply meant the absence of conflict, but I will argue that there is no difference for those who live with oppression.
Questions of Policy. These questions ask what should be done about a social or legal issue, as in the following example:
The way to decrease the rate of sexual assault on women is not to educate them on how to protect themselves, but to educate men on how and why to control their behavior.
Asking and answering thesis questions can help writers before they begin drafting, and can guide and inform their work throughout the writing process.
Source: Adapted from Sophia Instructor Gavin McCall