Borrowing from Frank Ramsey’s “On There Being No Discussable Subject”, the following is a opening to a similar question within political philosophy (hopefully the topic category will make this clear enough, but my intention here is not to cover the whole of philosophy).
As Ramsey did (kind of), I’m going to open with a possible list of things we can’t discuss.
1. Factual disputes
We are simply not in a position to adjudicate on matters of fact. Very rarely are political philosophers also experts on international relations, economics, medicine, environmental sciences etc., but even if we were, we’d be only of the same level of expertise as others in the field. We would not be of a higher level of expertise such that we might adjudicate on competing theories.
The common counter to this is that we can use logic and reason to adjudicate, but of course, we’ll all claim to be doing that, which just kicks the can further down the road - who then adjudicates on that dispute.
So matters of fact must be taken as unresolved where there is dispute on those facts among experts in the field.
2. Factual disputes masquerading as ethical ones
Relating to (1). There is a common class of discussion where overtly factual disputes are coerced into ethical ones to make them amenable to non-expert discourse. For example overbearing police tactics might be defended with “you wouldn’t want criminals out on the streets able to do harm to the vulnerable would you?”. The matter of dispute is clearly over the facts pertaining to whether those being targeted are, in fact, criminals and whether they are, in fact, putting vulnerable people at risk. Or the dispute may be over whether the police tactic in question does, in fact, reduce that risk.
What the dispute is almost certainly not over, is whether it is ethically permissible to deliberately allow vulnerable people to be put in harm’s way.
3. Historical outcomes
I don’t think there’d be any disagreement with the notion that we live in a very complex world with hundreds of factors responsible for any given state of affairs. It’s also evident that we’ve only run history once. We have a sample of one problem.
And yet is is quite a common topic of dispute that “we tried X and it turned out this way, therefore X is wrong”. People will be familiar with this from the classic argument against communism, but it also applies on a meso-scale to discussions about government policy. Each move in politics is a move embedded in it’s historical context, in its time and place, and cannot be readily extracted from that to extrapolate over wide-ranging generalisations about ideological principles.
This is not that same as saying we cannot learn from history, or apply historical outcomes to disputes over political philosophy, but that the mere existence of precedent does not in itself constitute an argument. It is half an argument. “A similar policy was tried in this historical example and the situation then was similar to the situation now in the following ways …”, is the full argument. And, as with (2), it is this second half that is more often the real matter of dispute, but with discourse masquerading as dispute over the first half (that the precedent did indeed occur).
4. The self-evidency of the status quo
That things are the way they are is trivially self-evident and isn’t something which argues against a position saying they may be otherwise.
A centrist, status-quo maintaining, argument is often treated as in an equal position to that of an ideological one, but they do not have the same evidentiary burden. Note - I’m going to assume here that the centrist argument is something like “things must be this way because of x, y, z…”, setting aside the possibility that someone might actually argue the status quo is genuinely ideal and what we would have even in utopia. So the argument is not about how things ought be, but about how things could possibly be.
The counter from the centrist to the idealist has to be of the form that things cannot be the way the idealist intends. The key here is that an argument claiming things must be such-and-such a way has a significantly different burden of proof to an argument claiming that things could be such-and-such a way.
Leaving…?
So what remains as a ‘proper topic’ for discussion in political philosophy? As common with much of the rest of philosophy - internal consistency, coherence, and usefulness.
To what extent is fitting political events to a particular framework more coherent an explanation than the alternative? Which political ideology is most useful for understanding some political event? How might political events be seen under different frameworks, and to what extent is that internally consistent?