As we have discussed, your thesis must state your main argument. If you could guarantee that a month after reading your essay, readers will remember and be convinced of only one sentence from it, what would it be? That is your thesis.
A thesis cannot be a statement of fact or observation. It must be an arguable point, something about which someone could rationally hold a contrary position. That does not mean you believe the contrary position to be as valid as your own — that would make your own position arbitrary — but that someone could hold a contrary position without immediately being dismissed as a crank by people who possess some knowledge of the topic.
Remember that for this essay, the thesis must be conceptual, not practical. Your thesis should thus be a statement that renders a judgment that defines the reality of a situation, not one that announces how to solve a problem.
An effective thesis is almost always a complex or compound-complex sentence, and the most effective approach is to put the most compelling clause — the one that makes your thesis intriguing — at the end. In other words, a thesis builds to a climax. For example, the initial clause may state reasons for the thesis (Because X), establish a point to which the thesis acts as a counter-argument (Although X), or identify the means by which something occurred or was achieved (Through X or By X). These are not the only possibilities, of course.
Beware the dreaded Five Paragaph Essay thesis. I have found that when I ask students for a thesis, their initial instinct a majority of the time is to write a sentence with exactly three subtopics named in it. This is a bad habit inculcated by the Five Paragraph Essay model, in which the writer names the topics of all three body paragraphs in the thesis itself. This is silly. Five Paragraph Essays are quite short. With longer essays, following that model means you end up with either a ridiculously long thesis (no one benefits from a thesis that goes on for half a page) or a thesis that names three of the subtopics in the essay but ignores the rest. Your thesis is not the place to name all of your subtopics; it’s a thesis, not an outline.
The thesis must be capacious enough so that the entire essay is relevant to it. The way to do that is not to name all of your arguments but to encapsulate them. That means writing on a higher level of the scale of abstraction without being so broad as to make the thesis vague. On the other hand, the longer a thesis goes on, the less focused and compelling it usually becomes. Balancing comprehensiveness with energy and pith is one of the biggest challenges you will face in writing a thesis.
Do not ever quote in your thesis. Your thesis is your main idea; if you cannot articulate your main idea without depending on a source, you don’t have a main idea!
As always, follow the Format Rules.