Suppose someone says, “to figure it out, you have to look at A, B and C”
Someone else says, “That leads to the result K.”
First person says, “Whoa, that can’t be right. Should be J. Maybe we should look at A, B and D?”
You want to reject the conclusion your logic leads to, so you want to change the steps in the logic.
The closest is reductio ad absurdum.
Motivated reasoning. You don’t like the consequence, so you’re trying any alternate reasoning to avoid that consequence.
As a formal fallacy, there is also appeal to consequences. But that’s a little different, since it involves straight up denying the conclusion because if true, it would mean terrible things.
So I’d go with motivated reasoning here.
I don’t think it’s reductio ad absurdum.
The idea isn’t that the person is trying different logic paths to make other results look silly.
It’s that the logic path took him to a reasonable result that he doesn’t like, so he’s trying to change the logic path to get a different, also reasonable, result that he does like.
“Motivated reasoning” or “result oriented reasoning” might catch it.
How do we know that the conclusion is reasonable but simply unwanted? Maybe somebody has set things up so that reasonable-sounding propositions will “logically” lead to a conclusion that most people will find offensive.
It’s OK to suspect “somebody has set things up” to make a proposition you like have offensive outcomes. We all have motivated reasoning from time to time.
If you think your proposition doesn’t really lead to offensive conclusions, you have to show how the facts and logic are different than commonly understood.
If you can’t do that, then you must accept that your favored proposition does, in fact, lead to offensive conclusions.
But you can’t go around saying that your conclusion only seems offensive because all the arguments have been rigged against you. That’s not logical.
To what extent are you describing this kind of behavior? It certainly seems like a similar complaint although you’re referencing a logic path and I’m referencing general philosophical principles.
For a mathematician it is contrapositive reasoning. You don’t like the conclusion that there are only finitely many primes, so you say to yourself, "Well if there were only finitely many primes, you could multiply them and add one. Hey that number might or might not be prime but it is not divisible by any of our known primes (always leaves a remainder of 1) so any prime divisor must be a new prime. Therefore that conclusion is false.
The contrapositive is also what I thought of. Basically, if A implies B, then (not B) implies (not A). In your case, you have
(A and B and C) implies K
But if J and K are mutually exclusive, we have
J implies (not K) implies (not [A and B and C])
and “not [A and B and C]” is the same as “(not A) or (not B) or (not C)”. In other words, at least one of the premises must be false.
Denial.
I’m not sure it’s a fallacy. If the logic of A, B, C and D are right, then there is no fallacy and J and K are both true, or else the logic is wrong and a fallacy or unwarranted premise exists in one or both arguments. But simply choosing one valid logical chain over another is not a fallacy.
Reductio ad absurdum is the disproof of an assertion by showing that it would lead to a false or ridiculous result, or the proof of an assertion by showing that it would lead to a false or ridiculous result if it weren’t true. The contrapositive is a form of reductio ad absurdum. That’s not what the OP is asking about.
I agree with Darren Garrison that it’s appeal to consequences. The fallacy is that an assertion can’t be true because it would lead to a result I don’t like (not one that’s false), or that an assertion must be true because I wouldn’t like the result of it being false (not that it being false would lead to a contradiction).
This type of reasoning is very common. For instance, I think it’s the basis of a lot of climate change denial: if climate change is true, I would have to accept a lot of changes to my lifestyle that I don’t like, so it must not be true. There’s no actual disproof; the idea is rejected because I don’t like the consequences.
I think that’s it.
It’s a textbook case of appeal to consequences.
“We lost the election because we pushed for higher taxes and voters didn’t want that” - but if someone is a proponent of tax raises, one may want to reject that - “Uh, let’s attribute it to something else instead”.
I agree. From the link:
In the OP, the person doesn’t like the results of A, B and C, so they reject C and go with D.
I think humans are hardwired to accept this.
“If my lover/SO/etc. is cheating on me, it’s too painful. They aren’t cheating.”
If you don’t like the argument, you can call it casuistry:
“The use of clever but unsound reasoning, especially in relation to moral questions; sophistry.”
(There is also a more technical meaning for the word)
Backpedaling