When one says that a thing is a "slippery slope", one is obliged to prove that there is a slope and that it is indeed slippery. Likewise, a thin edge of the wedge is only relevant if there is a substantial "wedge" attached to the thin edge, and if there is some motive force behind said wedge. One might also have to demonstrate that a part of the wedge isn't already in place, and that this is in fact a slightly fatter part of the wedge. In any case, once the wedge is fully driven in (or the end of the slope passed) some calamity ought to follow.
In either figure of speech, there are three components:
- A small movement in a dangerous or negative direction (an implicit instability; the thin edge)
- A near-unstoppable force in that same direction (lubricated gravity; the implicit hammer)
- A highly dangerous or negative result (crashing at the bottom of the slope; splitting whatever the wedge is in)
It's well worth being conscious of each of these components when you use these metaphors. Some examples to consider, many of which are taken from actual arguments:
- Mandatory Internet filtering
- Allowing papists in the Royal Navy
- Abandoning the Lord's Prayer in Parliament
- Using the proprietary software present in the firmware of a hotel elevator
- Giving money to a beggar
- Nationalizing banks
- Not shaving tomorrow