I deeply apologise for the length; I wanted to provide (over)sufficient context while questioning my confusion and miscomprehension. Please advise if this can be improved or pruned.
Source: The Legal Analyst, Ward Farnsworth
p 176: A final example involves the use, against people suspected of terrorism,
of coercive forms of interrogation—physical and mental pressure
that falls short of “torture,” though obviously there is a lot of room for
debate about where that line gets drawn. One argument against using
any such pressures at all is an appeal to the slippery slope: if we get used
to putting physical pressure on people, we will become numb to its horrors
and dangers and will be inclined then to move on to more aggressive
methods that would more obviously amount to torture; or we will
start using torture elsewhere—against people suspected of other things,
or against people convicted of crimes whom we want to punish regardless
of whether they have any useful information. (The “we” here might refer
to the police who do the pressuring or to the public at large.) All these
things could happen, but it is hard to prove that they will happen.
176. How is the above an example of a slippery slope? Farnsworth submits that 'All these
things could happen', so why does it matter whether they will happen or not? Aren't these all truly cogent, coherent reasons, regardless of their probability of occurrence? What's "slippery"?
p 177: A path of this kind, with the right to die expanded in each case,
was followed by courts in the Netherlands. The first decision by itself
might not have seemed to imply the last one, but each step between
them turned out to be slippery. This might have been so because norms
of equality—of treating like cases alike—made it hard to draw lines between
any two of them; in that case we might question whether it was
a true slippery slope or just a case where the implications of the first
decision weren’t fully appreciated until later. But it’s also possible that
distinctions between these cases are available and plausible yet hard to
draw in practice: each case presents heart-rending facts, and judges may
not have the stomach or, perhaps, the moral confidence to say that any
two of these situations are different enough to deny relief. If so, the result
could amount to a true slippery slope: a case where decision two really
was not thought to follow from decision one when decision one was
made, but did follow from it after all.
177. If in actuality, decision 2 did follow from decision 1, then how's this a slippery slope?
As an addendum, I thought to link to https://philosophy.stackexchange.com/a/4683.
Best Answer
The argument to a 'slippery slope' lies precisely in the inevitability which is claimed for the final undesirable outcome. Once you step onto such a slope its slipperiness is so great that you cannot achieve the traction or friction which would be needed to stop at some point along the way: you must continue all the way to the bottom.
In 176, "All these things could happen, but it is hard to prove that they will happen" is an argument against the slippery slope: the end is not inevitable, the slope is not so slippery that we cannot stop at some point short of the bottom.
In 177, "decision two" represents the bottom of the slope and "decision one" its top. Even though it "was not thought to follow from decision one when decision one was made", it did in fact "follow from it after all". Once decision one was made, decision two followed inevitably: that is "a true slippery slope".