> Argumentum ad populum [0] is itself an informal fallacy, as described on both of our links.
Which gets to the difference between one and the other.
"This is correct because everybody says it is" is a fallacy because it can be true or false independent of whether everybody says it is or not. Even if the premise is true, the conclusion can be false, or vice versa.
Whereas if the premises that X likely leads to Y and Y is bad are both true, then the conclusion that X likely leads to something bad is not independent.
> What I said wasn't an argumentum ad populum anyways: we're discussing definitions, and definitions do not have truth values.
Categories have definitions. Whether a particular thing fits into a particular category can be reasoned about, and a particular miscategorization being common doesn't make it correct.
> But not all incorrect premises are formulated in a reasonable manner. There are degenerate premises that have telltale signs of being misguided. These would be what make informal fallacies. In a way, you could think of them as being incorrect about the premises of what counts as sound logic.
The general form of informal fallacies is that they take some reasoning which is often true (e.g. if everybody believes something then it's more likely to be true than false) and then tries to use it under the assumption that it's always the case, which is obviously erroneous, e.g. the majority of people used to think the sun revolved around the earth.
The category error with slippery slope is that the probability is part of the argument. If 60% of the things people believe are true, that doesn't tell you if "sun revolves around the earth" is one of those things, so you can't use it to prove that one way or the other.
Whereas arguing that taking on a 60% chance of a bad thing happening is bad isn't a claim that the bad thing will definitely happen.