What's so fallacious about the Slippery Slope Fallacy?

Question

When you look at the world, I think it's a rather non-controversial statement that a good percentage, if not a majority, of social problems are caused by people making choices based solely on short-term outcomes--"if X then Y, and Y is desirable, therefore I will do X"--without taking long term consequences into account. (ie. Y leads to Z, which is more undesirable than Y is desirable, therefore you should not do X.) But in a recent discussion with a friend that got into this territory, when I tried to point out undesirable long-term consequences, he accused me of employing the Slippery Slope Fallacy and declared my reasoning to be therefore invalid, and that was the end of that. And it made me wonder. Why is it formally considered a fallacy to try to reason beyond a single degree of cause and effect, and why does this explicitly only apply when reaching a negative consequence? (I distinctly remember, when we learned about fallacies in high school, that "X leads to Y, which leads to Z, and Z is desirable, therefore you should do X" is *not* considered an example of the fallacy.) It seems just a little bit Orwellian to see the name of Logic invoked to formally dissuade people from one of the highest and most practical forms of logical reasoning, namely reasoning about the long-term consequences of one's own actions. So what am I missing? Why is this considered a fallacy?

Answer 1

Suppose Joe makes the following argument:

1. Every Lego in my Lego building is red.

∴ 2. My Lego building is red.

It's all too easy to accuse Joe of committing the fallacy of composition, which says that it's not generally the case that if a part of a whole has a certain feature then the whole itself has that feature.

A more charitable interpretation is that Joe's argument employs a hidden but reasonable premise, something like:

0. For any x and any y, if x is a Lego building, and y is a Lego piece, and x is composed of y, and y is red, then x is red.

Given a straightforward formalization of 0, 1 and 2, 2 would be a consequence of 1 and 2, and so Joe's argument would be valid. As such, Joe would be guilty of making a tacit but reasonable assumption. He would not be guilty of any fallacy.

All this to say is that it may be that you've committed no fallacy, but rather that your friend disagrees with a crucial premise in your argument and that because, without this premise, your argument would be guilty of the slippery slope fallacy, it's easier for your friend to accuse you of committing a fallacy than it is for him or her to confront your premises.

Perhaps we should call such a move "the Fallacy Fallacy"? What do you think? =D