The discussion around Optimism vs Mindset is very interesting and I am constantly trying to use both methodologies in tandem.

There are a number of ways to reconcile the two approaches. For example, one can note that much of what Dweck describes is a kind of Impostor Syndrome, where people start out with a negative impression of themselves, and have a fixed mindset about their lack of skill.

If someone has a belief that they are a terrible programmer, telling them they are a good programmer doesn't help. They simply dispute it, a well-understood phenomenon. For them, you must provide evidence to shake their permanent belief about their own shortcomings, and being specific is the way to do it.

But what if they are not already mired in self-doubt? That is another matter entirely.

Now here is something I try to apply. Note the difference between a skill related to production, and a "meta-skill," something that enables or powers other skills. One person I know told me that they are terrible at X and Y and Z in their profession, but that they have "grit," the ability to grind it out and get results relentlessly.

Note that they have a positive, permanent, and general belief about a skill, "grit," that isn't a direct producer of results, but enables results.

Another person told me that they reject general praise outright, because they know that they make a lot of mistakes in programming. So they want very specific feedback about what they did well, not general praise. You can guess what came next: They told me that they were not instinctively good, but that they could learn from feedback.

Aha! Another fixed belief about a meta-skill, learning.

And in fact, what Dweck advocates is learning to adopt a fixed mindset about the growth mindset! Another meta-skill. So perhaps, what works is encouraging fixed beliefs about skills that empower or enable the directly productive skills.

But I don't know. Unlike Seligman and Dweck, I am not testing and measuring.

I like the idea of shaking permanent beliefs/attitudes. From reading your talk, it sounds like taking the optimistic mindset is partly as an antidote to a pessimistic attitude, or to the idea that a person can be reduced to a (negative) rule.

The thing I worry about, however, is that this further entrenches the idea that there is anything that is permanent! If someone tells me I am good/bad at something, I think "well, I am good/bad at this particular thing in this particular way right now." General, permanent praise can ring hollow for me because it doesn't seem like something which can even "actually" be true, but at least I'm symmetric about this with criticism. (I also can't discount the fact that I heard so much general praise growing up that my brain tends to automatically ignores it as being likely ungrounded.)

I suspect adopting the growth mindset has an underlying mindset of impermanence, since if things aren't guaranteed to remain fixed, changing for the better is always a possibility.

Here's an anecdote I find interesting about people and how they view themselves: I was teaching an introductory calculus course, which tends to be full of freshmen, people generally known to be exploring their identities. After a quiz early on in the semester, a student came up to me, pointing to the score, with disbelief "how could this be? I am a good student!" There was a strong cognitive dissonance, and they were blinded because, I suspect, they didn't want to believe they might have been someone else this whole time. Exams don't know whether or not you are a "good student." (The student was bright and creative, and I think went on to do architecture instead.) (This was my first semester as a graduate student. I tried to handle the situation as well as I could given my role, ability, and right to interfere, but I'm no psychologist.)

Sometimes thinking about believing something just because it is emotionally beneficial reminds me of the following exchange from The Hitchhiker's Guide to the Galaxy (by everyone's favorite pessimist):

    Slartibartfast: I'd much rather be happy than right any day. 
    Arthur Dent: And are you? 
    Slartibartfast: Ah, no.  (laughs)

But it doesn't seem to me that rejecting positive/negative impressions of yourself as being who you are is unhappiness-inducing. If anything, it might reduce cognitive dissonance. Here's a scenario where this might fall apart: there is a skill you know you have deficiencies in, someone asks you to do something which requires you to put that skill on display, so you are hesitant, because you don't want to signal to people that you thought this was good work, since you know people might take this as indication that you are "bad" because people have generally bought into the whole permanence mindset, and then you'd have to spend more effort to manage how people perceive you, otherwise people might take you less seriously overall for future things you would like to do. If you're fine with perception management, then there's no issue, but otherwise there is some unhappiness because of an effective insecurity derived from the interaction between the secure self and the social sphere.

Anyway, to steal your signoff: But I don't know. Unlike you I haven't read Seligman and Dweck, and unlike them I am not testing and measuring.