The Semantic Slippery Slope is a fallacy that occurs when someone argues that because there is no clear line between two concepts or because they "only" differ in degree, therefore either they are the same thing or neither exists at all.
This fallacy is somewhat of an inversion of the False Dichotomy, in which someone ignores any grey area and posits that only two contrasts exist. The Semantic Slippery Slope emphasizes any grey area and disregards clear differences. In short, saying the concept is too vague for any real decision to be made. The Semantic Slippery Slope Fallacy is also related to the regular Slippery Slope Fallacy insofar as committing the former will often cause the latter by inferring that one thing will inevitably cause the second thing, or that they're the same thing altogether.
- This is also known as "Loki's Wager". In Norse Mythology, Loki wagered that two Dwarven blacksmiths could not make a particular weapon and offered his head as collateral. When the Dwarves succeeded, they demanded Loki's head, but he made their claim irrelevant by stating that they had won only his head, and that since there is no clear line where the head ends and the neck begins, they could not take the former without also taking the latter (they couldn't just take the top half of the head, since they won the whole head). The dwarves then proceed to sew Loki's mouth shut. The head was theirs, after all. Nothing in the deal about it having to be removed for that to happen. The term "Loki's Wager" has become used as an alternative name for this very fallacy.
- Also called the Fallacy of the Beard. Exactly how many hairs does a man have to have on his chin before he has a beard? What's so special about that number that X hairs means he doesn't have a beard and X+1 means he does? So therefore you can't tell if a man has a beard or not! (Except you usually can at a glance, without much argument from anyone else.)
- This is closely related to the Paradox of the Heap: taking a grain of sand away from a heap still leaves you with a heap, therefore, since you can keep taking grains of sand one by one, one grain of sand is a heap. The solution is to use fuzzy logic, where a heap can be defined with blurry boundaries, and taking away a grain of sand within a certain range makes it less likely to be a heap, and when the likelihood is zero it stops being a heap.
- Sometimes, you get an intimate partner (or a religious ideology) who believes that so much as looking at an attractive member of the relevant sex that isn't them is the same as climbing into bed with that person and thus cheating.
- This is used to save a character's life (and brutally screw over Shylock) in Shakespeare's The Merchant of Venice. It happens in a manner very similar to the Loki entry above: due to a wager between the two, Shylock is legally entitled to one pound of Antonio's flesh, and he intends to collect. Portia, disguised as a lawyer, manages to convince a court that Shylock can take the flesh, but can't take any blood without violating the agreement (and being punished with death). Worse, he can't simply back out, or he gets the same punishment.
- Also related to the Continuum Fallacy.