How interesting. I will have to think about that some more.

A couple of days ago on one of my mailing lists, a question (not aimed specifically at me) came through:  What do you say when someone tells you that global warming is a myth?  It's an interesting question, less so for its specific content than for its general case.  How do we articulate that general case? 

The general case is not "What do you say when someone tells you that [X] is a myth?",  where X is a belief that you hold to be true.  Global warming isn't a "belief," it's a single noun.  For [X] to be a "belief" (or a "myth" for that matter) it would have to be a complete sentence.  So Global warming as used in that construction really has to be a stand-in for some complete sentence, or even for an entire manifesto.  The speaker of the original statement (Global warming is a myth) is using shorthand, according to a definition or system of definitions that exists in her own mind, and that the hearer (who poses the question What do you say when someone tells you global warming is a myth?) needs to access in order to respond logically.

So the general case is really, "What do you say when someone tells you that [X] is a myth?",  where X is a term that, in the "someone's" mind, stands for a set of statements P = {P1, P2, ..., Pn}.

Here is where most hearers, and most people ready to answer the hearer's question "What do you say?", make an incorrect assumption: X in the speaker's mind stands for the same thing that it does in my mind (the hearer's mind), i. e., the set of statements Q = {Q1, Q2, ..., Qn}.

You can see where this is going to go.  Nowhere useful, unless it happens that there are a lot of {Px = Qy}.

That's why, when someone tells you "X is a myth," the first thing you have to say is "Tell me exactly what you mean by X."  And don't proceed until you are satisfied that you understand, not X, but P.

Let's get back to that global warming question.  The original speaker, who said, "Global warming is a myth," may have meant any of the following things or perhaps something else:

1. She might mean, "I do not believe that the Earth is warmer now on average than it was 100 years ago."
2. She might mean, "I do not believe that the Earth is warmer now on average than it was 10,000 years ago."
3. She might mean, "Although I believe that the Earth is warmer now on average than it was sometime in the past, I am not convinced that the temperature jump is statistically significant compared to the background noise of natural temperature variations."
4. She might mean, "Whether or not the Earth is warmer now on average than it was sometime in the past, I am not convinced that the cause is primarily man-made."
5. She might mean, "Whether or not the Earth is warmer now on average than it was sometime in the past, and whether or not the cause is primarily man-made, I do not perceive the danger to be significant."
6. She might mean, "Whether or not the Earth is warmer now than in the past, and whether or not the danger to me is significant, I do not believe that the solutions that are usually recommended are likely to make a difference."
7. She might mean, "Whether or not the Earth is warmer now than in the past, and whether or not the danger to me is significant, I believe that the solutions that are usually recommended are likely to be more harmful than beneficial."

Or she might mean some combination of those, or something else.  Perhaps the hearer disagrees with the speaker.  Perhaps the hearer does not.  But this question -- whether the hearer disagrees with the speaker -- is logically independent of whether P is equivalent to Q, i.e., whether the term X means the same thing to both interlocutors.

To find out if they agree or disagree, the hearer first has to find out from the speaker what is P, and mentally substitute it into the speaker's statement so that it becomes "P is a myth."  Then, and not until then, they can have an argument.  Assuming that they have anything to argue about.

But back to the hearer's question.  What do you say when someone tells you that [X] is a myth?  Supposing that P has been substituted and the hearer disagrees.  Two further considerations are: 

1. Does the hearer know any facts that are (this is important!) established by an authority that both interlocutors accept and that are relevant to her specific objections?
2. Does the hearer have any well-considered opinions that are relevant to her specific objections?

If not, that's the time to say "How interesting.  I will have to think about it some more."