But no one knows it when they see it! Alignment is famous for causing constant forum arguments and unending terminological confusion!
People on forums argue endlessly about what alignments "really" mean for the same reason that humans endlessly argue over whether Ideal Forms "really" "exist"; because we're insane.
It makes sense in-universe for people to have monstrously complicated philosophical and theological cruft around the nine alignments - people have that out of universe too, despite our advantage that alignment has only existed for thirty-odd years, and was invented in plain view for anyone who cares to actually look at sequence of events which caused them to say the words which are coming out of their mouth. The debate gets increasingly elaborate, but you don't need to pay any of it any mind because fundamentally no one can rationalize what is irrational.
Alignment was meant as a tool to aid in roleplaying, and - and I do think that it's useful for people who haven't played a whole nine characters and exhausted its depth. But it's a common observation that humans, being insane, roleplay better in systems that don't track alignment. I think it's because if they have an alignment written on their character sheet, their character models, uh, snap to grid, so to say - to the grid that exists in their mind, I mean, not to some kind of universal platonic grid.
If you live in a world where alignment exists, but you nonetheless want to be an entire human person, you should forget about Law and Chaos and just do what's right.