Is there such as thing as the "ill-informed opponent" fallacy?

It's interesting that you mention the Fallacy of Subjectivism of which I was unaware because I see the tendency of my identical twin to fall into that fallacy and I wonder why she does and I don't. For example, she'll say, "I can't believe something if it doesn't make sense to me," to which my response would be, "I can understand that but unless you have agreed-upon facts for others to work with why should anyone go along with your lack of belief because something doesn't make sense to you." I have no interest in my own opinion on something that is a fact-based matter if I want to convince others of what I think is true, I want to ensure I can point to relevant facts - at least what I believe to be facts.

It's so frustrating because facts really do prove things although certainly facts can sometimes lead one astray too - there is the question of determining the most relevant facts. The nature of reality is that all the facts one way or another will ultimately favour the correct hypothesis.

Perhaps you need to change your approach. 

In my experience, people never change their beliefs by being punched in the head with facts. Most people believe what they believe because they want to believe what they currently believe. Facts are not important. Michael Shermer made this addition to Cognitive Dissonance Theory in his book, “Why People Believe Weird Things.” So, if your goal is to change another person’s belief, I think you must use a different approach.

Peter Boghossian suggested a strategy to change a person’s belief. To be successful, he said the person whose belief you want to change must reconsider how he arrived at the belief under discussion. If your goal is to change his mind, as distinct from pontificating (which is better done in front of a mirror), then you need to get him thinking about how he arrived at the belief.  

Boghossian’s book, “How to Have Impossible Conversations,” is an excellent manual on how to do this. In my experience, people never change their beliefs by being punched in the head with facts. Most people believe what they believe because they want to believe what they currently believe. Facts are not important. Michael Shermer made this addition to Cognitive Dissonance Theory in his book, “Why People Believe Weird Things.” So, if your goal is to change another person’s belief, I think you must use a different approach.

Peter Boghossian suggested a strategy to change a person’s belief. To be successful, he said the person whose belief you want to change must reconsider how he arrived at the belief under discussion. If your goal is to change his mind, as distinct from pontificating (which is better done in front of a mirror), then you need to get him thinking about how he arrived at the belief.  

His book is a good investment in conversations, beyond your identical twin. 

[To Dr. Richard]

Could not agree more that most people believe what they believe because they want to believe whatever it is ... but my question is why? Why do people want to believe one thing rather than another, why not always simply want to believe whatever is true? There are a number of things I believe to be true which I find very unpalatable and would rather weren't true but if that's the reality so be it. If something is unpalatable we can understand people's reluctance to believe it and that is often what I believe to be people's obstacle in accepting something as true - its unpalatability but sometimes it doesn't seem as if the truth is unpalatable and yet people still don't want to believe it.

I'm perfectly happy to be punched in the head with facts because I recognise that it is facts that tell the truth but I realise that for other people it's not the same and quite often they have no desire to discuss the matter seriously so even if you don't try to bash them over the head with facts they're still very difficult to discuss the subject with.

EDIT: I didn't realise I'm familiar with Peter Boghossian from watching a YouTube where he conducts an exercise engaging students on campus on the topic of gender and gets a very hostile reaction from some. Very interesting exercise.

EDIT 2: Oh dear, his technique didn't work very well on this occasion. Oh my goodness. The thing is I think both sides make interesting points but the emotionality on one side destroys the discussion.
https://youtu.be/VUOP_lY7DT8

[To Petra Liverani]

The question of "why" enters the field of psychology. The list is long. If anyone can figure out a solution to that question, he will become a psychological hero. Unfortunately, as you note, and so does Boghossian, some people's beliefs are not amendable to change. I sure had that reinforced with my recent book www.deathbypitbull.com. As you noted about a Boghossian video, those people often become belligerent and attack with ad hominem comments. 

Often you can see how someone came to their belief, especially when you held the belief yourself at one point but came to see that the belief was false.

I mentioned unpalatability but I forgot there's another one that seems incredibly powerful - anchoring in one's belief. There are certain things I believed quite strongly but when I could see so clearly how I was led to that belief and that not only clear evidence but good reason required a change of mind I changed it no problem and yet I find other people struggle with this change of belief in a way I find truly unfathomable, they have such a strong attachment to their belief even when it is exposed as false from quite a number of angles ... and while I can understand this phenomenon when the required belief is unpalatable it is difficult to understand when it is not unpalatable.

A truth’s initial commotion is directly proportional to how deeply the lie was believed. It wasn’t the world being round that agitated people, but that the world wasn’t flat.

When a well-packaged web of lies has been sold gradually to the masses over generations, the truth will seem utterly preposterous and its speaker a raving lunatic.

​The ideal tyranny is that which is ignorantly self-administered by its victims. The most perfect slaves are, therefore, those which blissfully and unawaredly enslave themselves.

Dresden James

What you are describing, is, I think Festinger’s Question.

Festinger’s Question comes from his famous 1956 book, “When Prophecy Fails.” Suppose (1) an individual believes something with his whole heart and soul; (2) he has taken irrevocable actions because of it; and (3) he is then presented with evidence, unequivocal and undeniable evidence, evidence he himself fully accepts as true, that his first belief is wrong. Festinger’s question is: What will happen?

The answer, well documented by six decades of subsequent research, shows that people respond to dissonant beliefs using three key strategies. 

First, they can ignore the dissonant belief. In essence, saying, I don’t want to believe it. Therefore it isn’t true. This, as the psychologists would say, is a form of repression. This is pure subjectivism, holding the primacy of consciousness to be true instead of the primacy of existence.

Second, they can reduce the importance of the conflicting belief. This is evident by phrases such as “I’ll think about it tomorrow,” meaning I have more important things to consider. This, as the psychologists would say, is a form of evasion.

Third, they can make the newer conflicting belief consistent with the older existing belief by twisting the evidence, then claiming the beliefs are not really in conflict. This, as the psychologists would say, is rationalization. Michael Shermer calls it “motivated reasoning.” 

What Festinger did not expect was people did not question their beliefs. Quite the opposite. Researchers were astonished to find people became stronger in their irrational beliefs after having been presented with unequivocal and undeniable evidence the subject himself entirely accepted as true. For example, if they believed in the flat earth, then they were presented with the undeniable evidence of the spherical planet, they became firmer in their flat earth belief. Today, psychologists call this the “Backfire Effect.”

The most difficult beliefs for people to examine are those beliefs that have been (1) held for a long time, (2) adopted before the age of reason, and (3) most often repeated. 

This explains why it is impossible to have a conversation on the two subjects one should never discuss socially: religious and political beliefs. Both of which are drilled into children from the time they are born.

One may say, “every belief should be open to reexamination upon the presentation of credible evidence,” but attempting to live up to that standard is difficult and takes a concentrated effort.

[To Dr. Richard]

"Suppose (1) an individual believes something with his whole heart and soul; (2) he has taken irrevocable actions because of it; and (3) he is then presented with evidence, unequivocal and undeniable evidence, evidence he himself fully accepts as true, that his first belief is wrong. Festinger’s question is: What will happen?"

But sometimes no actions are taken and effectively nothing will change except the belief itself and the change in communication of that belief. And we can sometimes see that communication of the new belief should even be easier to achieve than communication of the old belief - but still the person resists the new belief seemingly purely from resistance caused by anchoring in their belief - that's the way it seems to me anyway.

So why don't we do a test, Dr Richard? Is there any mainstream dogma you've learnt is a lie and now that you hold the truth about it would have you labelled a crackpot?

I don't understand the paragraph beginning with: "But sometimes no actions are taken and effectively nothing will change except the belief itself and the change in communication of that belief"

What I mean is:

• there is no need for (2) (he has taken irrevocable actions because of [his beliefs]) for a person to resist changing their belief, eg, whether the earth is round or flat people don't behave differently, it's just a belief that has no bearing on how we conduct our lives and yet when it emerged that the earth was round and not flat people seemingly resisted the idea - as my quote says it wasn't the earth being round that was the problem, it was that it wasn't flat - if no one had had any notion about the shape of the earth prior to it being discovered it was round no doubt its shape would have been met with much less or even no resistance. It's more understandable, of course, if we have invested in our belief with actions that we might resist more the need to change it - the sunk-cost fallacy might be more inclined to influence our thinking - but I have noticed where it's just a matter of belief where actions play no part that people are still very resistant to changing that belief.
• in terms of communication, I can see that where people wish to communicate what they believe to be true but what they believe is a difficult belief for others to take on, if they modified their belief to what actually is true (their belief can be shown to be not quite right) they are still very resistant to changing that belief. They seem to give absolutely no thought to the fact that a modified belief might be easier for others to accept and therefore it's worth investigating the challenge to their belief to see if the evidence supports changing it.

The response to your statements here are in Festinger's book. For example, "A man with a conviction is a hard man to change. Tell him you disagree and he turns away. Show him facts or figures and he questions your sources. Appeal to logic and he fails to see your point."

Festinger,  Leon; Schachter,  Stanley. When Prophecy Fails . Start Publishing LLC. Kindle Edition.