The following was my answer to the of mixing metaphors question:
You’re right that the metaphors aren’t interchangeable. But I don’t think they’re giving us different pictures so much as different viewing angles on the same structural insight. The riverbed captures the relationship between what moves and what holds still and allows for gradual change. The scaffolding captures the enabling function. The axis captures the fixed point around which inquiry revolves. The hinge captures the fixed/moving relationship. Foundation and bedrock capture depth. So, they emphasize different things, stability, enabling, depth, the relationship between fixed and moving, but they converge around a single idea, viz., something stands fast, and it’s not part of our epistemic language. It makes epistemic language possible.
I don’t think hinge and foundation are incompatible. A hinge is what has to stay fixed for the door to swing. A foundation is what has to be in place for the building to stand. Both describe something that enables activity without participating in it. The metaphors come from different domains but they’re picking out the same structural role.
On your reading of hinges as propositions protected from questioning in a given context - I think that’s a bit too narrow. It captures something about how hinges function locally, within specific language games. But when Witt introduces the hinge metaphor in OC 341-343, he’s not talking about debate conventions. He’s talking about what has to stand fast for questioning to be possible. That’s not context-dependent protection. It’s the precondition of the practice.
Your boiling water example actually illustrates this well. The decision not to abandon the generalization and instead look for additional variables, that’s a hinge commitment in action. The framework holds fast and inquiry adjusts around it. That’s exactly how hinges function as foundations.
I think the riverbed metaphor handles this naturally. The water flows, the riverbed holds, and the riverbed itself sits in a wider landscape. Some hinges are local and can shift over time. Others, such as object persistence, experiential continuity, are bedrock in a way that “no one has been to the moon” never was. That’s not mixing metaphors. It’s recognizing that not all hinges sit at the same depth.
This was my response to Ludwig.