Apparently even shifting PC Alignment from G to E because they rob and murder would be railroading; if the player declares that murder and robbery is Good, then for the DM to declare otherwise is hashing his bliss.
In games like 1e-2e AD&D You can't eat babies, rape cabin boys, sacrifice puppies to Satan and still be classed as Good. That may be Railroading by Ron Edwards & Vince Baker's Forgeist definition, which seems some kind of Nietszchean "I am my own value-creator" idea, but not by mine or I think any reasonable definition.
If you are using alignment mechanics to stop other players taking the game into territory you're not interested in - murder, robbery (other than of orcs?), sacrifice, etc - then this would be exactly what I mean by sublimation of a metagame issue into an ingame issue. I personally don't find this a very effective way of resolving the problem.
If a GM doesn't want to run such a game, I don't see the benefit of saying "no evil PCs" as opposed to "we're going for heroic rather than brutal in this game".
But look upthread at the discussion of slavery - a pretty common fantasy trope, and we can't get agreement from those talking about it as to how D&D alignment rules are to treat it. If you don't want to play a game in which players own or capture slaves, then just say so. Conversely, if you want to make slavery a focus of your campaign (and I have done this in the past), then how is alignment helping, other than encouraging the GM to prejudge the issue and enforce that judgement on the rest of the table?
In D&D, at least by RAW, "evil" is more than a metaphysical concept. It is "real", and it can be detected. Whether or not something is "evil" in that sense can be a known factor within the context of the world.
The problem is that in D&D alignments ARE. (I'm not the first to bring this up.)
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The overall point is that the DM does need to determine what is a good act and what is an evil act as a truth.
Well, what you say is true of AD&D and 3E. It isn't true of 4e - the notions of "good act", "evil act" and "changing alignment" are not canvassed anywhere in the rules as I recall.
This is one reason why I prefer to play 4e rather than earlier editions.
I also totally agree that D&D is NOT the system for the style of play that I'm talking about.
I'd never, ever run this sort of game in D&D. It just doesn't work. Good and Evil in D&D are ontological. They aren't abstract in the slightest - they're actual physical forces just like gravity. To do this sort of narrativist game in D&D would require a HUGE amount of reworking of the mechanics.
In 4e this is not true at all. I know, because I'm doing it - running thematically-focused 4e without having to change the mechanics at all.
In AD&D it is not all that hard either - you just ignore the alignment rules, and then you have only the detect/protection from evil things to worry about (I can't remember exactly how I handled these back in the day).
I can't comment on 3E, other than to say that alignment mechanics seem more strongly built in (a wider range of alignment oriented spells, clerics with restrictions closer to those of an AD&D paladin, damage reduction keyed to alignment, etc).