Slippery slopes are themselves a slippery concept. Imagine trying to explain them to an alien:
"Well, we right-thinking people are quite sure that the Holocaust happened, so banning Holocaust denial would shut up some crackpots and improve the discourse. But it's one step on the road to things like banning unpopular political positions or religions, and we right-thinking people oppose that, so we won't ban Holocaust denial."
And the alien might well respond: "But you could just ban Holocaust denial, but not ban unpopular political positions or religions. Then you right-thinking people get the thing you want, but not the thing you don't want."
This post is about some of the replies you might give the alien.
Abandoning the Power of Choice
This is the boring one without any philosophical insight that gets mentioned only for completeness' sake. In this reply, giving up a certain point risks losing the ability to decide whether or not to give up other points.
For example, if people gave up the right to privacy and allowed the government to monitor all phone calls, online communications, and public places, then if someone launched a military coup, it would be very difficult to resist them because there would be no way to secretly organize a rebellion. This is also brought up in arguments about gun control a lot.
I'm not sure this is properly thought of as a slippery slope argument at all. It seems to be a more straightforward "Don't give up useful tools for fighting tyranny" argument.
The Legend of Murder-Gandhi
Previously on Less Wrong's The Adventures of Murder-Gandhi: Gandhi is offered a pill that will turn him into an unstoppable murderer. He refuses to take it, because in his current incarnation as a pacifist, he doesn't want others to die, and he knows that would be a consequence of taking the pill. Even if we offered him $1 million to take the pill, his abhorrence of violence would lead him to refuse.
But suppose we offered Gandhi $1 million to take a different pill: one which would decrease his reluctance to murder by 1%. This sounds like a pretty good deal. Even a person with 1% less reluctance to murder than Gandhi is still pretty pacifist and not likely to go killing anybody. And he could donate the money to his favorite charity and perhaps save some lives. Gandhi accepts the offer.
Now we iterate the process: every time Gandhi takes the 1%-more-likely-to-murder-pill, we offer him another $1 million to take the same pill again.
Maybe original Gandhi, upon sober contemplation, would decide to accept $5 million to become 5% less reluctant to murder. Maybe 95% of his original pacifism is the only level at which he can be absolutely sure that he will still pursue his pacifist ideals.
Unfortunately, original Gandhi isn't the one making the choice of whether or not to take the 6th pill. 95%-Gandhi is. And 95% Gandhi doesn't care quite as much about pacifism as original Gandhi did. He still doesn't want to become a murderer, but it wouldn't be a disaster if he were just 90% as reluctant as original Gandhi, that stuck-up goody-goody.
What if there were a general principle that each Gandhi was comfortable with Gandhis 5% more murderous than himself, but no more? Original Gandhi would start taking the pills, hoping to get down to 95%, but 95%-Gandhi would start taking five more, hoping to get down to 90%, and so on until he's rampaging through the streets of Delhi, killing everything in sight.
Now we're tempted to say Gandhi shouldn't even take the first pill. But this also seems odd. Are we really saying Gandhi shouldn't take what's basically a free million dollars to turn himself into 99%-Gandhi, who might well be nearly indistinguishable in his actions from the original?
Maybe Gandhi's best option is to "fence off" an area of the slippery slope by establishing a Schelling point - an arbitrary point that takes on special value as a dividing line. If he can hold himself to the precommitment, he can maximize his winnings. For example, original Gandhi could swear a mighty oath to take only five pills - or if he didn't trust even his own legendary virtue, he could give all his most valuable possessions to a friend and tell the friend to destroy them if he took more than five pills. This would commit his future self to stick to the 95% boundary (even though that future self is itching to try to the same precommitment strategy to stick to its own 90% boundary).
Real slippery slopes will resemble this example if, each time we change the rules, we also end up changing our opinion about how the rules should be changed. For example, I think the Catholic Church may be working off a theory of "If we give up this traditional practice, people will lose respect for tradition and want to give up even more traditional practices, and so on."
Slippery Hyperbolic Discounting
One evening, I start playing Sid Meier's Civilization (IV, if you're wondering - V is terrible). I have work tomorrow, so I want to stop and go to sleep by midnight.
At midnight, I consider my alternatives. For the moment, I feel an urge to keep playing Civilization. But I know I'll be miserable tomorrow if I haven't gotten enough sleep. Being a hyperbolic discounter, I value the next ten minutes a lot, but after that the curve becomes pretty flat and maybe I don't value 12:20 much more than I value the next morning at work. Ten minutes' sleep here or there doesn't make any difference. So I say: "I will play Civilization for ten minutes - 'just one more turn' - and then I will go to bed."
Time passes. It is now 12:10. Still being a hyperbolic discounter, I value the next ten minutes a lot, and subsequent times much less. And so I say: I will play until 12:20, ten minutes sleep here or there not making much difference, and then sleep.
And so on until my empire bestrides the globe and the rising sun peeps through my windows.
This is pretty much the same process described above with Murder-Gandhi except that here the role of the value-changing pill is played by time and my own tendency to discount hyperbolically.
The solution is the same. If I consider the problem early in the evening, I can precommit to midnight as a nice round number that makes a good Schelling point. Then, when deciding whether or not to play after midnight, I can treat my decision not as "Midnight or 12:10" - because 12:10 will always win that particular race - but as "Midnight or abandoning the only credible Schelling point and probably playing all night", which will be sufficient to scare me into turning off the computer.
(if I consider the problem at 12:01, I may be able to precommit to 12:10 if I am especially good at precommitments, but it's not a very natural Schelling point and it might be easier to say something like "as soon as I finish this turn" or "as soon as I discover this technology").
Coalitions of Resistance
Suppose you are a Zoroastrian, along with 1% of the population. In fact, along with Zoroastrianism your country has fifty other small religions, each with 1% of the population. 49% of your countrymen are atheist, and hate religion with a passion.
You hear that the government is considering banning the Taoists, who comprise 1% of the population. You've never liked the Taoists, vile doubters of the light of Ahura Mazda that they are, so you go along with this. When you hear the government wants to ban the Sikhs and Jains, you take the same tack.
But now you are in the unfortunate situation described by Martin Niemoller:
First they came for the socialists, and I did not speak out, because I was not a socialist.
Then they came for the trade unionists, and I did not speak out, because I was not a trade unionist.
Then they came for the Jews, and I did not speak out, because I was not a Jew.
Then they came for me, but we had already abandoned the only defensible Schelling point
With the banned Taoists, Sikhs, and Jains no longer invested in the outcome, the 49% atheist population has enough clout to ban Zoroastrianism and anyone else they want to ban. The better strategy would have been to have all fifty-one small religions form a coalition to defend one another's right to exist. In this toy model, they could have done so in an ecumenial congress, or some other literal strategy meeting.
But in the real world, there aren't fifty-one well-delineated religions. There are billions of people, each with their own set of opinions to defend. It would be impractical for everyone to physically coordinate, so they have to rely on Schelling points.
In the original example with the alien, I cheated by using the phrase "right-thinking people". In reality, figuring out who qualifies to join the Right-Thinking People Club is half the battle, and everyone's likely to have a different opinion on it. So far, the practical solution to the coordination problem, the "only defensible Schelling point", has been to just have everyone agree to defend everyone else without worrying whether they're right-thinking or not, and this is easier than trying to coordinate room for exceptions like Holocaust deniers. Give up on the Holocaust deniers, and no one else can be sure what other Schelling point you've committed to, if any...
...unless they can. In parts of Europe, they've banned Holocaust denial for years and everyone's been totally okay with it. There are also a host of other well-respected exceptions to free speech, like shouting "fire" in a crowded theater. Presumably, these exemptions are protected by tradition, so that they have become new Schelling points there, or are else so obvious that everyone except Holocaust deniers is willing to allow a special Holocaust denial exception without worrying it will impact their own case.
Summary
Slippery slopes legitimately exist wherever a policy not only affects the world directly, but affects people's willingness or ability to oppose future policies. Slippery slopes can sometimes be avoided by establishing a "Schelling fence" - a Schelling point that the various interest groups involved - or yourself across different values and times - make a credible precommitment to defend.