At tables with adequate communication and reason, this wouldn't be an issue.
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There will always be gaps. The problem could always arise; communication seems a stronger solution, to me.
I don't see why communication and reaching a table-wide agreement about what "good" means in this campaign is stronger than not insisting on a table-wide usage of "good". The latter allows everyone to play their characters, and for the GM to handle backstory and setting, as s/he thinks is appropriate - and the collisions between these things help drive the actual play of the game.
For me a comparison would be this: the PCs meet a dragon and decide to try and defeat it in combat. Do they succeed or not - ie is it true or false in the fiction that the PCs beat the dragon? One way would be for everyone to sit around, think hard about what the PCs can do and what the dragon can do, communicate frankly about this, and reach agreement. Another way would be to
play the game. I prefer playing the game to work out what happens to the dragon; and I prefer playing the game to work out whether or not a particular player's evaluative conception of his/her PC, as expressed through the play of that PC, is viable/worthwhile/good/bad/admirable/despicable. In my experience the results can be interesting.
I don't think the alignment system restricts the way you play your character though, it just offers a picture of how the universe responds to it.
But that the universe responds in that way itself has implications for the truth or falsehood of certain moral and cosmological conceptions. For instance, if the universe is neutral as between being good or being evil - as is the case in Planescape, where both are valid alignments able to shape the planes by way of belief - then the typical mindset of a paladin (according to which the universe is on the side of good, and providence will ensure that honour and duty align to reinforce rather than oppose one another) has already been refuted, and the paladin is consequently self-deluded.
As I said upthread, this can work in some games - eg in a Conan game a paladin
would be self-deluded - but doesn't work for the sort of fantasy game I default to, which is romantic fantasy a la Tolkien and King Arthur, not REH-esque.
It seems to me that there is little challenge in role playing this character's strong moral directives when the player knows, and the mechanics dictate, that the character is always more likely to succeed by adhering to those moral beliefs than if he deviates from them. What is difficult is maintaining principals when one would benefit by compromising or ignoring those beliefs.
This paragraph begins by talking about the player - and whether it is hard or easy to roleplay a particular character - and then ends up talking about the PC - who is maintaining certain principles. Talking in that way already involves many assumptions about playstyle that are not true for the way I play or GM the game. For instance, I don't expect my players to find it hard to roleplay their PCs being challenged. I hope that they will find it easy and enjoyable, and challenging only in the sense of setting an intellectual and aesthetic goal to aspire to - but in that respect, playing your PC being rewarded can be equally challenging, although often less dramatically engaging.
I submit that, in the real world, the tenets of a person's religion (much less the views of a deity) are not set by that person.
Once again you seem to be confusing the player with the PC. Most people don't get to decide who their parents were either, but players frequently do that for their PCs. Heck, GMs set the whole cosmology in which their NPCs live, but no one supposes that all of the NPCs are therefore gods who got to create their own universe!
How is it that everyone can agree without question what constitutes "honourable" and "dishonourable", but if we label something "Lawful Good", suddenly no possible agreement can exist?
They don't. That's part of the point of playing a paladin - finding out what your own conception of "honour" might require, and finding out what happens when that comes into collision with other participants' conceptions.
I responded to your specific statement that these were For example of play to which an alignment system would be an impediment, see the two provided earlier in this post. I do not agree that, objectively, an alignment system would be an impediment to good play of those scenarios. You did not say it could be an impediment, or that it would be an impediment to some players, but that it objectively and universally would be an impediment.
A poster - was it [MENTION=6706967]Dwimmerlied[/MENTION]? - asked for example of play in which alignment had been an impediment. I gave some. I didn't describe scenarios that other's might play using alignment rules. I described examples of play. Part of those examples - inherent to them - is the experience that I and the other participants had in playing them.
The second example - about the paladin turning on the heavens - at various points along the way incorporated material from the WotC scenario "Bastion of Broken Souls". I'm sure there are some people who could, and did, enjoy playing that module using alignment rules as part of the game. I know that I couldn't, and hence I didn't. Hence I am an example of a play experience which would have been impeded by the presence of alignment rules.
It may or may not improve your personal play experience, but that was not the question you posed. It would not improve the play experience for someone who wants his character to be a pawn on the chessboard, always making the best tactical move his player desires, rather than a living, breathing character with strengths and weaknesses either.
Actually, as I posted upthread, alignment rules seem to have been invented precisely for players whose PCs were pawns on a chess board. They introduced an extra constraint on the play of those pawns, in return for better access to certain benefits (like hirelings, healing and resurrection).
Conversely, once you are playing living, breathing characters why do you need alignment? You just play your character. If one of your character's "weaknesses" is that s/he won't fight dirty then just play him/her that way - though the whole idea that not fighting dirty is a weakness rather than a strength strikes me as wildly misguided unless the focus of the game is on nothing but extracting benefits from others via the most efficient application of physical force.
Who makes that call? The player, the player with the rest of the table (but without the GM), or the table including input of the GM?
This question doesn't really make sense to me.
No one has to make the call. Who gets to decide whether Denethor is evil, or simply misguided? No one does. Each reader is called upon to make his or her own judgement. For that matter, who gets to decide whether Tolkien's ultra-conservative vision of human affairs is admirable or not? Each reader is, again, called upon to make his or her own judgement.
Likewise in the games I run. Did the paladin who turned on the heavens and established a new cosmic order do the right thing or not? I'm pretty sure the player though that he did. If other participants though differently, that's there prerogative. No one needs to "make a call".
In another game, when a PC, depressed by the death of his only true love, agreed to sell out his home city in return for the promise of a magistracy, was the PC doing the right thing or not? The player didn't think so - the player was playing his PC's collapse into moral degeneration. I think the player of the other PC who procured the sell out might have taken a different view. No one needs to "make a call", though - we just play the game, see what happens and enjoy the experience.
Ultimately, alignment is about stipulating, in advance, what sort of behaviour is required to be a virtuous person. One of my main interests in fiction is being pushed to think in new ways about what a person might be and do. I think it's obvious that these two things are not compatible - that prior stipulation of what virtue requires is an obstacle to thinking in new ways about what a person might be and do. Alignment therefore being an impediment to my preferred play, I don't use it.