Logical fallacy
Informal
Vagueness
Slippery Slope
Alias:
•Argument of the Beard
•Fallacy of the Beard
Quote…
…[I]f once a man indulges himself in murder, very soon he comes to think little of robbing; and from robbing he comes next to drinking and Sabbath-breaking, and from that to incivility and procrastination. Once begin upon this downward path, you never know where you are to stop. Many a man has dated his ruin from some murder or other that perhaps he thought little of at the time.
…Unquote
Source: Thomas De Quincey, “Second Paper on Murder”
Exposition:
There are two types of fallacy referred to as “slippery slopes”:
1.Causal Version:
Type:
Non Causa Pro Causa
Form:
If A happens, then by a gradual series of small steps through B, C,…, X, Y, eventually Z will happen, too.
Z should not happen.
Therefore, A should not happen, either.
Example:
If today you can take a thing like evolution and make it a crime to teach it in the public school, tomorrow you can make it a crime to teach it in the private schools, and the next year you can make it a crime to teach it to the hustings or in the church. At the next session you may ban books and the newspapers. Soon you may set Catholic against Protestant and Protestant against Protestant, and try to foist your own religion upon the minds of men. If you can do one you can do the other. Ignorance and fanaticism is ever busy and needs feeding. Always it is feeding and gloating for more. Today it is the public school teachers, tomorrow the private. The next day the preachers and the lectures, the magazines, the books, the newspapers. After [a]while, your honor, it is the setting of man against man and creed against creed until with flying banners and beating drums we are marching backward to the glorious ages of the sixteenth century when bigots lighted fagots to burn the men who dared to bring any intelligence and enlightenment and culture to the human mind.
Source: Clarence Darrow, The Scopes Trial, Day 2
Analysis
This type is based upon the claim that a controversial type of action will lead inevitably to some admittedly bad type of action. It is the slide from A to Z via the intermediate steps B through Y that is the “slope”, and the smallness of each step that makes it “slippery”.
This type of argument is by no means invariably fallacious, but the strength of the argument is inversely proportional to the number of steps between A and Z, and directly proportional to the causal strength of the connections between adjacent steps. If there are many intervening steps, and the causal connections between them are weak, or even unknown, then the resulting argument will be very weak, if not downright fallacious.