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This is a conclusion based on the premise that if A happens, then eventually through a series of small steps, through B, C,..., X, Y, Z will happen, too, basically equating A and Z. So, if we don't want Z to occur, A must not be allowed to occur either. Example:
If we ban Hummers because they are bad for the environment eventually the government will ban all cars, so we should not ban Hummers.
What is Slippery Slope