This brings us without pause straight to the question of paladins and alignment spells. These seem to be a bone of contention even more so than alignments themselves. When arguments against alignment are brought up, the most frequent complaint is that the universe itself recognizes alignments; there are spells and abilities that affect different people based on their alignment, which many think of as silly. Since we've brought the paladin into this, though, we will also have to discuss a few misconceptions that people have about that class in regards to alignment and behavior.
The Paladin. An easy class to misrepresent. They have, through the poor playing of decades, a reputation of being uptight assholes that kill whatever they perceive as evil or wrong. They are the worst interpretation of Spanish inquisitor and Dominican monk that players can concoct. They are the morality police, mean and violent, and prone to destroying that which they disagree with. Right? Wrong. This is a paladin played incorrectly. Again, we must return to the source material to discover why.
The paladin is a noble and heroic warrior, the symbol of all that is right and true in the world. As such, he has high ideals that he must maintain at all times. Throughout legend and history there are many heroes who could be called paladins: Roland and the 12 Peers of Charlemagne, Sir Lancelot, Sir Gawain, and Sir Galahad are all examples of the class. However, many brave and heroic soldiers have tried and failed to live up to the ideals of the paladin. It is not an easy task!
Lawfulness and good deeds are the meat and drink of a paladin. If a paladin ever knowingly performs a chaotic act, he must seek a high-level (7th or more) cleric of lawful good alignment, confess his sin, and do penance as prescribed by the cleric. If a paladin should ever knowingly and willingly perform an evil act, he loses the status of paladinhood immediately and irrevocably. All benefits are then lost and no deed or magic can restore the character to paladinhood: He is ever after a fighter. The character's level remains unchanged when this occurs and experience points are adjusted accordingly.
We run up against two problems here very quickly; that is A) paladins must be LG, and B) a paladin who willingly violates his alignment is no longer a paladin. These are both the source of some consternation, but since we are discussing alignments primarily we shall address that issue first.
What does it mean to be lawful good? According to the AD&D PHB (which spends a great deal more time on alignment than any of the subsequent publications), a lawful good character is someone who generally:
...believe[s] that an orderly, strong society with a well-organized government can work to make life better for the majority of the people. To ensure the quality of life, laws must be created and obeyed. When people respect the laws and try to help one another, society as a whole prospers. Therefore, lawful good characters strive for those things that will bring the greatest benefit to the most people and cause the least harm. An honest and hard-working serf, a kindly and wise king, or a stern but forthright minister of justice are all examples of lawful good people.
Lawful good characters adhere to both the principle that order is important, and altruism is. While a lawful neutral character might follow the letter of the law, the lawful good character believes that the spirit of the law is more important. This is integral. Paladins are not unthinking murdermachines that apply their own rigid moral code to everything they see and, finding that it doesn't measure up, murder it. If they were, they wouldn't be paladins as the definition of the class suggests! Paladins must be forthright, honest, and merciful in their dealings. This is what it means to be strictly LG with no deviations.
What about falling? Surely everyone has heard of how paladins fall. It's a terrible thing to play a paladin, because the DM will force you to fall through no choice of your own! Never play a paladin unless you want the unenviable choice between eating a baby or watching ten million people die!
The only paladin that falls is one that willingly and knowingly commits an evil act. This is important. If you commit an evil act with full knowledge of what you are doing and do so under no compulsion, then you are effectively choosing not to be a paladin anymore. When a demon appears and says "You cannot attack me but you must choose: kill a baby or kill the kingdom?" whether you choose to kill the baby, the kingdom, or not to answer, none of these are evil choices. While the paladin may feel terribly guilty for a long time and require a special penance to regain his powers (which would be shaken by his lack of faith and the stain of his guilt) there are guidelines in place for questing to regain them.
Lastly, for the paladins alignment affecting abilities: how do they function? How does the world, the uncaring universe, see good and evil? There is indeed a metaphysical aura about characters who are strongly aligned; in this case, that means characters who are high level and deeply committed to their alignments. A high level CE cleric has led a life of such violence and evil that it clings to him like a second skin, and this is something paladins can see.
However, in AD&D detect evil is actually detect evil intent. Unless a character radiates villainy from the weight of their repeated crimes (a serial killer, a lich, an adventurer who kills other adventurers for profit) detect evil intent does not single them out. A man who is selfish does not detect as evil. A man who is intent on an evil act (he's planning on stabbing you in the future for no reason, he just came from a rape, he is plotting to assassinate his grandmother for money) will detect as evil. This is all explained clearly in the Complete Book of Paladins:
So we can see here that detect evil intent is not a dowsing rod for bad people. Nor can a paladin act on simply seeing an evil aura; LG precludes attacking someone because they MIGHT COMMIT a crime—this falls into a dark spectrum of LE. So indeed, the paladin could be forewarned or wary, or even discover that someone is a lich, vampire, or polymorphed evil dragon, but to attack them for an intention is simply not enough. (Of course, in a fantasy world one can safely assume that a vampire or polymorphed evil dragon has more than an intention and is a threat that must be eliminated, but a soul-searching chat with your DM would be a good idea before you draw your holy sword and attack; after all, it could be an illusory alignment created by a spell!)A paladin can detect evil radiated by characters and monsters; undead created by evil magic; Negative Plane influences; evil artifacts; certain enchanted swords; and other intelligent objects that radiate evil. The ability can't detect cursed objects or traps, nor does it work on creatures of Animal intelligence or less (Intelligence 0 or 1), such as centipedes or carnivorous plants.The paladin's sensitivity to evil responds to the target's intention to commit an evil act. The ability doesn't reveal the precise nature of the intended act, nor does it reveal the target's actual alignment. Characters who are strongly aligned, who do not stray from their faith, and are of at least 9th level might radiate evil if intent upon appropriate actions. For instance, if the paladin uses this ability on a suspicious nonplayer character, the paladin may sense that the NPC radiates evil, but not that the NPC is neutral evil, or that the NPC plans to ambush and kill the paladin. If an NPC recently murdered a passerby, the paladin might pick up evil emanations from the NPC but cannot determine the nature of the crime. Creatures such as the rakshasa, who disguise themselves with illusions, may conceal their appearances but not their evil intentions.A high-level character unshakably committed to an evil alignment may radiate evil even when not specifically planning an evil act or thinking evil thoughts. Powerful evil monsters, such as red dragons and hill giants, also radiate evil uncontrollably. A paladin can always detect the presence of these types of evil beings, unless unusual conditions are in effect. For instance, in some evil strongholds or planes, everything reads evil, effectively negating the paladin's evil-sensing ability.
So, alignments perhaps are not as metaphysically weighty as they first appeared. They essentially generate a weak aura that some items and spells interact with; those who are not unflinchingly committed to their alignments may not even do that. So chill out, friends, sit back, and use alignments to their full extent. Enjoy what they are, not what you think they are.
And for those who say that alignment is a simplistic philosophy I challenge you thusly: What is the nature of good? Is there no conversation worth having about it? In terms of D&D it is a devotion to altruism which the gods themselves recognize as a moral marker... but there are other, more complex things at play that determine a character's personality. If you are using LG or NE as shorthand for anything other than a very general way a character behaves, you are using them wrong. But in a pinch they can be excellent guidelines for how your character feels, since we must strive to inhabit someone who is not ourselves and understand their thoughts.
Or, you can dismiss this out of hand and laugh to yourself about how much better the world is without alignments. After all, alignments aren't real. And neither are gods, wizards, or heroes.