HISTORY STUDENT: If you'll forgive me, I'm not convinced that it is valid to talk about "political" philosophy unless there is some kind of practical policy application. It would be sort of like arguing about automobile philosophy in the absence of a practical application to cars. It might well be useful for something, but it wouldn't be automotive, and I could hardly blame a mechanic for completely ignoring my insights.
ME: I see the role of the political philosopher as figuring out what is valuable in terms of justice. Before you can make policies, you have to know what is worth pursuing. It sounds easier than it is. Philosophers look for what is valuable, and an ambitious philosopher will even try to determine some sort of ranking for what is valuable (with arguments that would/could appeal to any rational being, etc). More often than not, they don't get that far. And that's okay. Why? Because it's better to figure out what is valuable, if anything is, than not. Philosophers can argue about values, and prioritize them, and even try to get the proportions of importance correct without knowing exactly how things should be actualized in the real world. However, as you can imagine, this is a daunting task, and it takes a complex strategy to even attempt such a feat. Philosophers try to decide *what is good, or what helps people flourish (flourishing is haphazardly used a lot, and I don't like that, but does have some sort of basic meaning that everyone can grasp)? They construct arguments. Your question is an important question, one that philosophers talk about alongside these issues -- though rarely with complete answers.
So think of it this way. Maybe I don't know how to get Florida. Maybe I have no clue. Maybe I'm pretty sure I should head West. If someone can at least tell me to start going South, than I'm in a much better position than I was before. I'd be even better if they knew what states I should pass through, or even better what roads. You seem to think policy makers already know what justice is and what's valuable, but that's just not the case. I'm not sure I could tell you. But that doesn't mean I don't think there is an answer to be found out.
"Political" in political philosophy is a misnomer (if it weren't, I wouldn't like it). It's about explaining/understanding justice, not politics, not even the government. So really it's just a branch of ethics that focuses on justice over other values. And within ethics it's hard to make trade-offs (political philosophy is the most practical, I think; metaphysics might be the least).
After all, what do historians contribute to politics?
Example: Could we reasonably have a discussion about the morality of slavery in the absence of historical evidence about slavery's effects on real people? We could try, I suppose, but it would be highly unwise. People can justify (or condemn) anything as long as they can defer examination of that bottom line. In/justice isn't something that happens to propositions or ideals; it is something that is lived out by people.
For exactly the same reason, I am highly suspicious of anyone's attempt to discuss justice without discussing the means by which it is to be secured. In a perfect world, so to speak, everybody would have a pony. But I am going to benefit far more from the work of somebody who does not recognize the central importance of ponies yet makes ponies possible, than from the work of somebody who understands that ponies are essential yet fails to provide me with any means of getting one. The one makes justice possible (albeit not inevitable); the other merely talks about justice while the real world waits desperately for its ponies.
Sure, it is possible to do both -- to talk about ponies and deliver them, too. But the two things cannot be assumed to go together or even to be compatible with one another. If I miscalculate in the course of a practical campaign to deliver justice (as, say, Mao or Robespierre or G.W. Bush did), I may very well make the world worse than if I had left people alone to pursue justice in their own slapdash ways.
ME: All well and good, but I don't think you quite get it yet. What you're saying already depends upon a specific concept of justice. You're talking about consequentialism, at a minimum, but you have no way to decide which consequences are good and which are bad (though it sounds like you might go with utility --and you should be strongly suspect of utilitarianism). Sure, you have inclinations, but you haven't given me reason to believe that the effects of slavery, for example, are wrong in any sense of the word. You could argue that it's inefficient perhaps, but you haven't given a concept of morality with which to condemn it (or even to way to judge the effects). To measure something you need a ruler. To judge something you need to understand the standard that it's being judged against. Before you can compare the heights of two people, you have to have a concept of tall, and what it means to be taller. The standard can't merely be left to whim or intuition (though our intuitions can help us understand it), or even to what has historically been considered morally right. So you do your work with preconceived notions of morality, some of which may be right (after all, I do believe our intuitions are able to tap into an actual moral truths -- but not everyone believes this). You just aren't giving a concrete way to make those judgments.
How did you even figure out that it would be better for everyone to have ponies? You didn't. You guessed. You have to support it with an argument that is at the same time divorced from specific circumstances, and still responsive to facts about human nature and society (so no, we don't turn a blind eye to historians, sociologists, political scientists, psychologists, etc).
Even though other political leaders have pursued injustice, or justice in a misguided way (if you can even call it that), does not mean justice isn't a worthy goal to pursued by a collective force (over our individual slapdash ways).
So you say justice or injustice relates to people, and I agree, only I see them as moral agents operating against an actual moral standard (which is not a person on some views, but I believe the actual standard to be God, so perhaps I believe the standard is also an agent, I'm unsure) -- a standard that has principles independent of historical context (though not independent of the nature of the agents themselves). Human abilities to act morally remain unchanged ( at least since we acquired a conscious moral thought that conflicted with survival and instinctual response, if you like). We can do good or bad. Be selfish or altruistic. Pursue the excellent or pursue nothing at all. Yes, the outcomes have been different over time, and in some ways the same, but studying the outcome isn't enough, not without a standard to go by.
Continue to part two.