Philosophy
Epistemic Closure and the Regress Problem
Quick fact
The principle of epistemic closure is often used by skeptics to argue that if you don't know you're not a brain in a vat, then you don't know you have hands.
Why this is interesting
You know that Paris is the capital of France. But do you also know that the capital of France is the capital of a country in the European Union? If you know the first, and you know the second follows, do you automatically know the second? What if you never thought about it?