Of all the absurd and irritating arguments that people insist on foisting on me, perhaps the most ridiculous is the idea of Universal Law, or, for those of you who haven't kept up with your Kant, the 'what if everyone acted / thought like that' argument. People are always using this against me: "What if everyone chose not to get married?" "What if everyone quit their corporate jobs and went out and started helping the poor in Africa?" "What if everyone decided not to own / build a house?" "What if everyone wanted to use the bathroom at this precise moment?" "What if everyone you tried using this argument with punched you in the face? Would that make you stop?"
The main problem with this argument is precisely that most of the cretins who use it have have never actually read Kant in their life, and so what they're working off is a hazy notion of the concept of a Categorical Imperative without really understanding what that is. This is like using a fork to spread butter on your toast or a knife to drink soup. The point is that Kant's notion of Universal Law is really a notion for criminal law, a matter of fundamental ethical principle. Whatever the flaws in Kant's reasoning (and I believe there are many) he certainly did not intend the idea of Universal Law to be used to guide every single decision an individual makes in his life. The idea was to use the conecept of Universal Law to derive certain basic principles that would enable men to interact with each other - a sort of common minimum program, not unlike Hobbes' social contract. Using the Categorical Imperative to decide whether to kill a man who has tricked you out of all your property is one thing, using it to decide whether to go clubbing or sit at home reading a book is something else again.
Why should Universal Law not be applied to everyday situations? I can think of four reasons:
First, implicit in the argument for the Universal Law is the belief that there are some values / beliefs that are constant across all human beings, so that if everyone were to apply this law they would all come to the same conclusion. This may be true for very basic decisions - we would probably all agree that being murdered on the way back from work is a bad thing - but is unlikely to yield consistent results when the concept is over-extended. Not understanding this is the reason that people who use this argument are often surprised by the responses they get. (For instance, every time I tell people I don't want to have children they say "what if everyone thought like that? The human race would die out" My reply to this is that personally I don't know that this would be such a bad thing, and I certainly don't care if it happens)
Second, to the extent that the argument is made for things that are under human / social control the argument is tautological and represents nothing more than a failure of the imagination. The problem is that people usually end up doing partial analysis - they try imagining a world where all social institutions and arrangements are the same, with only this one difference in human behaviour. Unfortunately, social institutions are emergent and co-determined so that to really see what would happen if everyone thought a particular way requires an analysis on the scale of Plato's Republic. Take the argument as applied to marriage. If everyone decided not to get marriage, the problem would not be that there would be no social order or stability - it simply wouldn't be the social order that exists now. Society doesn't function because people get married, people get married because society functions and drives them to it - if no one were to get married we would soon evolve alternate social institutions that would allow us to go on with the business of living, raising children, etc. without scaffolding ourselves to some obsolete arrangement. A similar argument applies to development / socialism - if everyone truly understood and followed these ideas, then we could have a much better world since the economic and social arrangements (and therefore the desires and aspirations of individuals) would adjust accordingly. The trouble happens precisely because some parts of the system don't change when others do, throwing the whole thing into disequilibrium. Ironically enough, it is precisely where everyone thought as we did that our ideas would work, if they don't work it's because most people don't think like us.
A third problem with the argument is that it ignores the principle of division of labour. To the extent that specialisation is necessary for technological and social progress, we need people to act differently. So for instance, I would certainly not want to live in a world where everyone was a management consultant, but this does not mean that consultants do not have their place in society (okay, so it's under that green rock, next to the lawyers, but still). Most people are comfortable with this argument at an economic level (you'll rarely find anyone saying "What if everyone in the world decided to become a doctor"), but for some reason we're unwilling to extend it to social life . One could argue, for instance, that it might be beneficial to society to have some people who did not get married / have children and who were thus free to contribute in a meaningful way to human thought and art . Indeed, the fact that society may not sufficiently incentivise such behaviour can be seen to be a failing of the social market. Or put another way, would you buy a red dress if everyone else was going to be wearing exactly the same red dress the next day?
A closely linked idea is the fact of human variety. The whole point is that everyone is NOT going to think alike, that we will all have different desires and needs and abilities, and the best thing for society would be to let these different preferences co-exist and sustain each other. Everyone in the world is not going to give up their corporate job and start helping orphans in Darfur - there are enough people who dont' care enough about oppression and who like their plush offices and fat pay-checks - Wall Street will survive. The argument for Universal Law as applied to such individual decisions is a red-herring, a convenient hyperbole. The real question to ask is not "What if no one had children?" but rather "What if all those people who despised children and couldn't stand the little no-neck monsters didn't have children?".
A fourth (though related) issue is that (unlike ethical considerations ) many social decisions imply shared and constrained resources. So that applying Universal Law may simply not be feasible. To use a simple reductio ad absurdum, would you really want to be in a small book store buying a copy of the latest Harry Potter if every other person in New York City were there with you, crammed into that 600 square foot space, reaching for the exact same book? Or to put it differently, how would you feel if every single person in the world married your wife?
The truth is that the Universal Law argument is a crutch used by people who are insecure about the decisions they have made. That other people could live their lives any other way is a fact that these people find threatening, because it implies that they may have made the wrong decision for themselves. Unable to see any real logic in their own actions, they fall back on an imagined world where no one does what they did and the world falls apart as a consequence. This is fiction, of course, but it is a useful fiction, partly because it allows these people to pretend that they are 'responsible' and 'far-sighted' (of course you were only thinking of the growth rate of the GDP twenty years from now when you married this loser; the fact that you're a spineless moron who couldn't stand up to society if it was a wet paper bag has nothing to do with it)  and partly because if they can just force everyone else to do the same things they've done, there will be no way of telling that they were mistakes.
 My favourite response to this argument comes from Catch-22: "What if everyone thought like that?" "Well, I'd be a damn fool to think any other way, then, wouldn't I?"
 Ayn Rand makes the argument for suicide as a Universal Law - the idea that you could have a perfectly consistent world where everyone believed that the best thing for mankind was to die out
 For an interesting discussion on this see Durkheim. Durkheim argues that society exists as the balance between two forces - a centripetal force in the form of basic criminal law and ethical principles that holds us together and a centrifugal force for differentiation which helps to ensure the continued growth and evolution of society
 Think about it, how many great thinkers / writers do you know who were happily married when they wrote their great works? Surely these are the exception, rather than the rule. What was it Shelley said: "Most wretched men are cradled to poetry by wrong / They learn in suffering, what they teach in song".
 I won't even get into the question of whether it makes sense for the individual to sacrifice himself for society to that extent.