[A]ny thoughts on the implications of the following Popper quote: "No rational argument will have a rational effect on a man who does not want to adopt a rational attitude."I believe this quote embodies an Essentialist and a Foundationalist mistake. It is not true that, in essence, people do or do not want to adopt a rational attitude. They are mixed about it.
In an debate, characterising your opponent in such a manner would be specious meta. Outside one, there is value in making note of which people seem better than others, to decide who to interact with and how. But such theories should be left behind for actual interactions, where what matters is the content of the interaction and not meta categorising.
As to the Foundationalist error, it is the mistaken theory that if one lacks the proper foundation (in this case, a rational attitude) then one will not have any views logically consistent with that foundation. But people are not perfectly consistent, and the Foundationalist could be wrong about what is logically consistent with what. And these two effects are compounded the further distant the so-called foundation and the thing in question are.
In this case, Popper seems to think that what someone wants (explicitly?) with regard to rationality will dictate whether they act rationally. But it seems to me the structure of one's worldview, and the deep, inexplicit mechanisms for changing it and generating conjectures and criticism, have little to do with what one wants. How one reacts to external criticism has something to do with one's wants, but certainly is not dictated by them.
For the sake of precision, I'd like to harp on one thing I wrote. I phrased the Foundationalist claim as a person not having views consistent with a foundation he lacks, instead of implied by.
I wanted to avoid taking a stance on which way the implications go (or if they even go a direction at all). If we view a set of theories as a one-way chain anchored to the ground, in makes twofold sense to expect to lose the chain when we lose an early link. First, because the chain above the link is no longer anchored to the ground, it would fall away. And second, because the implications only go up the chain, losing a link leaves the next link stranded (it is no longer implied by anything). But if we then remove that link on those grounds, the next suffers the same fate, and so on along the entire chain.
A better view is chain-links that attach on two (or more) sides, and need no anchor. We get theories by bold conjecture, and keep them if they beat their rivals. No where in that process must we justify (anchor) the theories. So it does not matter if they seem to float in the middle of nowhere, as long as they contain good explanations.