a.k.a. "the three degrees of cluelessness".
It's an unfortunate fact that there is a strong tendency for people to lump these three things together, mainly because their observable effects tend to be prettty much identical... the making of spectacularly stupid statements.
The problem is if you don't differentiate between someone saying something stupid because they've never been taught any better, someone saying something stupid because they're genuinly not bright, and someone saying something stupid because they've decided they have a preferred view of the world and to hell with any pesky facts that contradict it... then you've basically decided that you're not in the business of changing anyone's mind about anything. The status quo is just fine with you. For example, this guy appears to have no interest in doing anything but hurling invective at people on the internat all day. Does he need to differentiate between the ignorant and the stupid to do so? Well, no. I'm not sure exactly what he gets out of doing what he does, but whatever.
For those of us interested in actually teaching people things however differentiating between those three classes of people matters. Because an alternate set of terms for them is:
Teachable, Teachable With Difficulty, and Not Worth The Effort.
If you are responding to every stupid infactual statement someone makes by calling the person who makes it a retarded fuckwad you are driving away those that might be educated with even a little effort. And if the existence of people who make stupid statements is irritating enough to you that it evokes that kind of response one would think you had a legitimate interest in reducing their numbers. Which means at least engaging with the teachable, if not the "teachable with difficulty".
At first glance it can be hard to tell the difference between these three types of people of course. Some individual came storming in to your discusion of the evidence for the age of the earth ranting about how the magnitic field of the planet is decaying therefore the earth HAS TO BE YOUNG!!!!
Are they saying that because nobody has ever actually taught them that the manetic field fluctuates constantly, and even sometimes reverse itself? Are they saying that because people have tried to teach them that but they're too stupid to understand the concept? Or are they saying that because they work for Answers In Genesis and they spend their entire existence thinking up ridiculous reasons to believe the bible is literally true and have no interest in any data that says anything else? Who knows? You can't tell from the statement itself.... you're going to have to exchange at least a few sentences with this person to find out.
If the first one is "you're a brain dead moron" the only thing you're goin to find out is that people react with hostility to being called a brain dead moron. If on the other hand your first sentence is "I think you may be unaware that the earth's magnetic field actually does this..." you are better positioned to provoke different responses from the three different groups of people.
The ignorant (btw, don't call them that) may not immeditately believe you but should be open to having it demonstrated to them.
The stupid may not be able to grasp what you just said but might be reachable if you can figure out a way to simplify the subject.
The delusional will have a tendency to declare that that's just what the evil evolutionist conspiracy wants us to think and declare you hate Jesus... or something along those lines at least. You can write these people off. At this point feel free to call them brain dead morons if that does something for you.
Once you have some idea what you're dealing with you can decide how to proceed. If you have a relatively intelligent person who just doesn't have the information required to properly understand the subject, give it to them! Do it respectfully, without implying they're an idiot for not knowing it already, and you have decent chances of reaching these people.
If you have someone who just doesn't seem to have the brain power to handle the subject it's trickier (and more depressing). It really depends on exactly how well YOU know the subject, and what skills you have in simplifying complex subject matter without distorting it, which is harder than it sounds. You may want to make the attempt, you may not. Your call. If you don't however... PLEASE try not to poison the well for someone down the road who might want to try teaching this person. Just let them know you don't think they're getting the full picture and disengage... don't get in a screaming match with them about how they're too stupid to discuss this with.
If you have a deluded person, the discussion is your amusement park. I really couldn't care less how you play with them. Enjoy!