There are a number of implicit concepts I have in my head that seem so obvious that I don't even bother verbalizing them. At least, until it's brought to my attention other people don't share these concepts.
It didn't feel like a big revelation at the time I learned the concept, just a formalization of something that's extremely obvious. And yet other people don't have those intuitions, so perhaps this is pretty non-obvious in reality.
Here’s a short, non-exhaustive list:
* Intermediate Value Theorem
* Net Present Value
* Differentiable functions are locally linear
* Theory of mind
* Grice’s maxims
If you have not heard any of these ideas before, I highly recommend you look them up! Most *likely*, they will seem obvious to you. You might already know those concepts by a different name, or they’re already integrated enough into your worldview without a definitive name.
However, many people appear to lack some of these concepts, and it’s possible you’re one of them.
As a test: for every idea in the above list, can you think of a nontrivial real example of a dispute where one or both parties in an intellectual disagreement likely failed to model this concept? If not, you might be missing something about each idea!