If you have a thesis, your first sentence will be your thesis.
If you don't have a thesis, it so happens that you run out of writing time before you can figure out an overall, final answer to the question as you understand it at this time, you might have a "takeaway", which is a short statement or summary of where you are at the point you start writing. This may be an explanation of why you can't make up your mind at this point, or a question you think needs answering, or what you think you need in order to go on and figure out a thesis. Your "takeaway" is thus your biggest, or most significant, or most comprehensive overall point about the subject. So, if you don't have a thesis, your first sentence should be your takeaway. Your "takeaway" could be a single question, such as "I think the main question is ___________ " or, if it's more than one question, you could say something like "I have several questions. They are ______________" If you don't even have a takeaway, and you should resort to the thinkathon option. Or your takeaway could just be "I don't really have a handle on this topic just yet, so I'm just going to do a thinkathon, which is just a collection of whatever random thoughts I happen to have about the topic".
Copyright © 2022 by Martin C. Young