The graduate handbook says that in order to embark on a doctoral dissertation, you have to turn in a dissertation proposal, and defend it in a meeting of your dissertation committee. Here are guidelines that proposals have to satisfy if I'm going to direct the dissertation, and guidelines for preparing for your proposal defence. They also indicate what I'll be looking for as a committee member, even when I'm not the primary advisor. (N.B.: If I haven't both seen a proposal and been in a meeting of the committee where it has been approved, then I'm not on your committee, and I won't sign off on the dissertation. Don't list me as a member of your dissertation committee if this stage hasn't been completed!)
Your proposal will normally be on the order of 15 pages. (The graduate handbook gives a shorter estimate -- I think 5 pages -- and you're welcome to try to cover the bases listed below in that space, but the higher page estimate matches my own experience of what it takes to do the job.) It should explain what your problem is, what you propose to argue for, and roughly how you propose to argue for it (a description of your strategy) -- and why all of that is philosophically interesting. I take that last point very seriously; I won't sign off on a proposal that doesn't take time out to explicitly motivate your project.
In the course of doing this, you will need to briefly frame your project with respect to the current state of play. This can mean giving a taxonomy of the currently held views about your problem, indicating which players occupy which position in your taxonomy, and where your own view would fit into it. It can mean describing how the debate about your problem has evolved, and how your dissertation is the right next move. Either way, you should convey a sense of who your dissertation is meant to speak to, and how it will advance an ongoing philosophical debate.
Finally, the proposal should come with a short bibliography. Your bibliography says this: "here are the readings that frame my approach to the problem, with the implication it's okay for you, the committee, to ask me about them, and I'll have something to say about them and about how I think they fit into the story I'm telling."
Overall, your proposal should show that your dissertation project is both ambitious and realistic. (Click here for a model dissertation by a recent U of U student.) The proposal itself doesn't need to contain a lot of argumentation. Rather, it has to make it plausible that the arguments will turn up as you write the dissertation. You can help convince the committee of that by having some argumentation on hand when you get to the meeting. (One thing I am likely to ask you in the proposal defense is: okay, give me an example of how these arguments are going to go, just so we can see what you have in mind.) That means that, even though the arguments don't go into the proposal, you shouldn't put off generating them until after the proposal has been approved; on the contrary.
If it's a dissertation for which I'm the primary advisor: I have a policy that governs my classrooms, namely, "no sex, no politics, no religion," and it applies to dissertations as well. If your proposal describes a dissertation project that enters one of these topic areas, you will need to find a different advisor. And, heads up, if the dissertation is in progress, and it strays onto these topics, I will remove myself from the role.
Bear in mind, in deciding when to turn your proposal in, and also when to schedule the proposal defense, that you have to give the members of your committee time to read it over and think about it. You can't hand it to them two or three days before the meeting. A couple weeks lead time is probably optimal.
When your proposal is approved, I will inform the DGS, and request that you provide a copy of the proposal for the DGS's files.