Epistemic status: Preliminary thought that didn't fit in a tweet

I know at least one person who doesn't believe "common sense" is a real thing. I disagree, in the sense that within a given culture, there exist a pool of beliefs and a pool of epistemic operators that are approximately common knowledge, so when you present your neighbor with some new fact, it's approximately common knowledge between you two that they will make all of the obvious (1-step) inferences from the pool of beliefs plus the new fact, using the pool of epistemic operators. Maybe they won't do this instantly or automatically, but if you can assume they're motivated to think about it, they'll draw at least some predictable conclusions. I call the pool of shared operators "common sense", and the inferences "common sense beliefs".

Note that I'm using "common knowledge" here in the technical sense that, e.g., two parties Alice and Bob know what each other knows, and they each know that each other knows, and they each know that each other knows that they know, etc., etc., ad infinitum. It's approximate in two senses: parties are computationally limited so they don't actually have infinitely recursive common knowledge, and the individual facts about which they have this approximate knowledge aren't exactly the same between them. The practical effect is the same though. I can be pretty sure, for example, that if you're reading this blog post you know English. I also know that you know I know that. And I can be pretty sure you know that I know that you know that I know... Seems crazy, but helps us make unconscious inferences we would be otherwise unable to make.

Anyway, here's a sketch of an argument for the existence of those two pools: Because we all have biologically pretty similar brains and homogenizing public schools followed by consumption of the same social-reality-creating major media outlets seem likely to produce a pool of common knowledge. We can have common knowledge of certain inferences "everyone" will likely draw from the knowledge commonly available to everyone again because our brains are pretty similar, and because if there weren't such a pool I don't think we'd even be able to have a conversation in the first place: We wouldn't even know what lines of argument to suspect others would find cogent.

NB: I'm not saying everyone will agree on what's common sense, only that they'll mostly agree. I'm also not saying that they'll agree what lives in each of those pools, only that they'll mostly agree. I'm also not saying that more well-informed and well-trained people don't have larger pools - they do - though I'd expect they still have common knowledge of