Ideally the knowledge we have should be true, that’s what I think, anyway. One way that our conclusions might be invalidated is if we allow our choices to be clouded by emotion. I’m not talking about removing all emotion from our lives, nobody wants an army of robots instead of the human race; there are plenty of times when emotion should be what guides us. When it comes to objective, empirical truths, however, we should try to keep emotion out of it.
The Argument From Final Consequences, aka the Appeal to Consequences, is when you let your heart overrule your head because you like or dislike the consequences. It can be written thusly:
If A is true then it will lead to x
x is desirable therefore A is true
The opposite can also be true with x having undesirable consequences and A therefore being untrue. A common real world example of the fallacy is:
If there was an afterlife then I would get to live forever
Therefore there is an afterlife
Clearly no amount of wanting to live forever will actually make it so. The actual evidence of an afterlife has not been evaluated, it has merely been stated that it is desirable (which, personally, I don’t think it is).
An example in the negative would be if a creationist argued that evolution was not true because humans would be no better than animals if it were so. Just because you don’t like something it doesn’t make it not so. The example I hear every week from my girlfriend is: having lung cancer sounds awful, therefore her smoking isn’t doing her any harm. Unfortunately, the carcinogens in her lungs are a selfish bunch and couldn’t give a figgy pudding about her desires.
The Argument From Final Consequences, then, is fallacious because it appeals to your emotions. Please don’t do it, it’d make me sad.