Epistemic status: Hortative. I'm trying to argue for carving reality at a new joint.
I think it's lovely and useful that we have labels, not just for rationalist, but for rationalist-adjacent and for post-rationalist. But these labels are generally made extensionally, by pointing at people who claim those labels, rather than intensionally, by trying to distill what distinguishes those clusters.
I have some intensional definitions that I've been honing for a long time. Here's the biggest one.
A rationalist, in the sense of this particular community, is someone who is trying to build and update a unified probabilistic model of how the entire world works, and trying to use that model to make predictions and decisions.
By "unified" I mean decompartmentalized- if there's a domain where the model gives two incompatible predictions, then as soon as that's noticed it has to be rectified in some way.
And it's important that it be probabilistic- it's perfectly consistent to resolve a conflict between predictions by saying "I currently think the answer is X with about 60% probability, and Y with about 25% probability, and with about 15% probability I'm missing the correct option or confused about the nature of the question entirely".
The Sequences are aimed at people trying to do exactly this thing, and Eliezer focuses on how to not go horribly wrong in the process (with a special focus on not trusting one's own sense of obviousness).
Being a rationalist isn't about any specific set of conclusions- it's not about being an effective altruist, or a utilitarian, or even an atheist. It's about whether one is trying to do that thing or not. Even if one is doing a terrible job of it!
Truth-seeking is a prerequisite, but it's not enough. It's possible to be very disciplined about finding and assembling true facts, without thereby changing the way one thinks about the world. As a contrast, here's how the New York Times, whose fact-checking quality is not in dispute, decides what to report:
By and large, talented reporters scrambled to match stories with what internally was often called “the narrative.” We were occasionally asked to map a narrative for our various beats a year in advance, square the plan with editors, then generate stories that fit the pre-designated line.
The difference between wielding a narrative and fitting new facts into it, and learning a model from new facts, is the difference between rationalization and rationality.
"Taking weird ideas seriously" is also a prerequisite (because some weird ideas are true, and if you bounce off of them you won't get far), but again it's not enough. I shouldn't really need to convince you of that one.
Okay, then, so what's a post-rationalist?
The people who identify as such generally don't want to pin it down, but here's my attempt at categorizing at least the ones who make sense to me:
A post-rationalist is someone who believes the rationalist project is misguided or impossible, but who likes to use some of the tools and concepts developed by the rationalists.
Of course I'm less confident that this properly defines the cluster, outside of groups like Ribbonfarm where it seems to fit quite well. There are people who view the Sequences (or whatever parts have diffused to them) the way they view Derrida: as one more tool to try on an interesting conundrum, see if it works there, but not really treat it as applicable across the board.
And there are those who talk about being a fox rather than a hedgehog (and therefore see trying to reconcile one's models across domains as being harmful), and those who talk about how the very attempt is a matter of hubris, that not only can we not know the universe, we cannot even realistically aspire to decent calibration.
And then, of course:
A rationalist-adjacent is someone who enjoys spending time with some clusters of rationalists (and/or enjoys discussing some topics with rationalists), but who is not interested in doing the whole rationalist thing themself.
Which is not a bad thing at all! It's honestly a good sign of a healthy community that the community appeals even to people for whom the project doesn't appeal, and the rationalist-adjacents may be more psychologically healthy than the rationalists.
The real issue of contention, as far as I'm concerned, is something I've saved for the end: that not everyone who self-identifies as a rationalist fits the first definition very well, and that the first definition is in fact a more compact cluster than self-identification.
And that makes this community, and this site, a bit tricky to navigate. There are rationalist-adjacents for whom a double-crux on many topics would fail because they're not interested in zooming in so close on a belief. There are post-rationalists for whom a double-crux would fail because they can just switch frames on the conversation any time they're feeling stuck. And to try to double-crux with someone, only to have it fail in either of those ways, is an infuriating feeling for those of us who thought we could take it for granted in the community.
I don't yet know of an intervention for signaling that a conversation is happening on explicitly rationalist norms- it's hard to do that in a way that others won't feel pressured to insist they'd follow. But I wish there were one.
To some extent I mean both things, though more the former than the latter.
I'll give a direction answer, but first consider this not perfect comparison that I think gives some flavor of how it seemed to me the OP is approaching the post-rationalist category such that it might evoke the feeling in a self-identified rationalist the sort of feeling a post-rationalist would have seeing themselves explained the way they are here.
Let's give a definition for a pre-rationalist that someone who was a pre-rationalist would endorse. They wouldn't call themselves a pre-rationalist, of course, more likely they'd call themselves something like a normal, functioning adult. They might describe themselves like this, in relation to epistemology:
A normal, functioning adult is someone who cares about the truth.
They then might describe a rationalist like this:
A rationalist is someone who believes certain kinds of or ways of knowing truth are invalid, only special methods can be used to find truth, and other kinds of truths are not real.
There's a lot going on here. The pre-rationalist is framing things in ways that make sense to them, which is fair, but it also means they are somewhat unfair to the rationalist because in their heart what they see is some annoying person who rejects things that they know to be true because it doesn't fit within some system that the rationalist, from the pre-rationalist's point of view, made up. They see the rationalist as a person disconnected from reality and tied up in their special notion of truth. Compare the way that to a non-rationalist outsider rationalists can appear arrogant, idealistic, foolish, unemotional, etc.
I ultimately think something similar is going on here. I don't think this is malicious, only that orthonormal doesn't have an inside view of what it would mean to be a post-rationalist and so offers a definition that is defined in relation to being a rationalist, just as a pre-rationalist would offer a definition of rationalist set up in contrast to their notion of what it is to be "normal".
So yes I do mean that "in order to give a satisfying constructive definition of post rationalists, one must give up commitment to a single ontology" because this is the only way to give such a definition from the inside and have it make sense.
I think the problem is actually worse than this, which is why I haven't proffered my own definition here. I don't think there's a clean way to draw lines around the post-rationalist category and have it capture all of what a post-rationalist would consider important because it would require making distinctions that are in a certain sense not real, but in a certain sense are. You might say that the post-rationalist position is ultimately a non-dual one as way of pointing vaguely in the direction of what I mean, but it's not that helpful a pointer because it also is only a useful one if you have some experience to ground what that means.
So if I really had to try to offer a constructive definition, it would look something like a pointer to what it is like to think in this way so that you could see it for yourself, but you'd have to do that seeing all on your own, not through my words, it would be highly contextualized to fit the person I was offering the definition to, and in the end it would effectively have to make you, at least for a moment, into a post-rationalist, even if beyond that moment you didn't consider yourself one.
Now that I've written all this, I realize this post in itself might serve as such a pointer to someone, though not necessarily you, philh.