There are two simple rules that we learn very early as schoolchildern. First, everyone is entitled to their opinion. Second, opinions cannot be wrong. Unfortunately, this seems to be understood by many people to mean "Anything that I think is my opinion. This means nothing I say can be contradicted because opinions can't be wrong and any attempt to argue is depriving me of my right to an opinion." In practical terms, that means that many arguments look something like this:
You pronounce the 'g' in 'gnat'
No, look at the pronunciation guide in this dictionary
Well, in my opinion, you pronounce the 'g'
Except that the 'g' really is silen-
No, it's my opinion and opinions can't be wrong
Sadly, I have had numerous arguments that have run according to that script both online and off over the course of my life. Now, before I even get into the subject of the comparative validity of differing opinions, I feel I should cover one of the basics of language. Positions can be broken into two different categories, opinions and facts. Opinions make an assertion about subjective qualities of reality. Facts makes assertions about objective qualities. Facts are not opinions. Facts can be wrong. If your position is that an untrue fact is correct, then you are wrong. If you find yourself uttering the phrase "Well, that's my opinion and I'm entitled to it." or something of that ilk in an argument about whether or not something really happened, it means that you lost.
Also of note is the fact that, contrary to elementary school lessons, not all opinions are created equal. Now, you would think that the later lesson that is usually reinforced in high school, that any argument that you can back up is good, would overwrite the previous one, but it seems this doesn't take in everyone. If your opinion, for example, is that Barack Obama is the worst president we've ever had, you are entitled to that opinion. If you believe he is the worst president because he is a foreign socialist Muslim who wants the terrorists to win, then your opinion, which you are still entitled to, is neverthelss completely moronic. (And, just to be less partisan, if you think Bush was the worst president ever because he has big ears, messes up pronunciations and blew up the world trade center, then your opinion is equally moronic). Now, you could hold either opinion and not be a moron if the reasons you hold that opinion are true, but if you are having an argument, you have to present those reasons. "He is the worst president in my opinion and therefore I am right because it is an opinion. Also, all my reasons are my opinions so they are equally not wrong." is how stupid people attempt to win (or at least avoid losing) a debate.
Don't be stupid.