It is best if you can capture your thesis in a single sentence. If you can do this, make it sentence #1 of your dissertation, and repeat this sentence, word for word, wherever you need to drive home the point of your dissertation. This is a tremendous aid in focussing your work. A side benefit is that it provides an unassailable defense to an entire class of attacks on your work. For example, should someone attack your work by pointing out that it does not scale, you simply reply,You may be correct, but right or wrong, your point is irrelevant. My thesis is that "crossbreeding gerbils with hamsters provides an order of magnitude speedup over standard treadmill technology." I clearly demonstrate factors of 12-17 in my dissertation; I make no claims beyond an order of magnitude. This is one of the benefits of focus.
Some examplesWhen I wrote my dissertation, I began with the opening sentence: Control-flow analysis is feasible and useful for higher-order languages. Then I spent 200 pages explaining first how to do CFA for higher-order languages (feasible), and second, the kinds of optimisations it enables (useful). My dissertation was nominated for the 1991 ACM Distinguished Dissertation award.
The first chapter of John Ellis' dissertation, Bulldog: A Compiler for VLIW Architectures, is titled "My Thesis." Not much room for misinterpretation here - it's clear what the chapter is all about. The first sentence of this chapter is:Ordinary scientific programs can be compiled for a new parallel architecture called VLIW (Very Long Instruction Word), yielding order of magnitude speedups over scalar architectures. There is never any doubt in the reader's mind what Ellis is setting out to demonstrate with his book.
Ellis' dissertation received the 1985 ACM Distinguished Dissertation award. I highly recommend it to anyone who wants to see how to write academic prose: it is the single best piece of academic writing I have ever read. It is clear and lucid. It does not get tangled up in stilted, passive, jargon-laden "academic" style. Ideas flow effortlessly off the page and into your head. When something is an opinion, it's obvious that it's an opinion; when something is a fact, it's obvious that it's a fact. You should try to write a dissertation this good.
The first sentence of Henry Massalin's dissertation on the Synthesis operating system is,This dissertation shows that operating systems can provide fundamental services an order of magnitude more efficiently than traditional implementations. He then spends 140 pages showing how this can be done. Henry's dissertation was nominated for the 1992 ACM Distinguished Dissertation award.
The point is: what are you trying to show? The point is: what is your point? If you can get that straight in your head, and put it up front at the beginning of your document, you will be able to proceed in a straight line. You will know what things are essential, and what things are distractions or detours. You will know when to stop writing: when you have demonstrated your thesis. If your thesis committee makes unreasonable demands of you, you will be able to tell them: "(a) My thesis, as stated, is a solid advancement of the field, and (b) I have supported my thesis. This is all I need to do to graduate; your requests are above and beyond this threshold. Cancel them and give me my degree."
Don't be alarmed if you are unable to precisely state your thesis when you start work in your thesis area - you may only have a general and long-winded notion of the problem and its solution. But you may find it useful, as you progress in your work, to refine this down to that single sentence (or couple of sentences) that states your thesis. As you grind away on your PhD, and your understanding of your problem matures, it will help you to have a little background voice asking at regular intervals: "What is my thesis?"