Axiom of Choice

A mathematical (and philosophical) theory. Introduced in the early 20th century, it states that if you have a bunch of stuff, you can pick one thing from that stuff, and if you have many different groups of things, you can select one from each group.

This was controversial at the time.

Math is weird.

I have a two bowls of Cap'n Crunch. It took mathematicians until 1904 to figure out that I can take a Crunchberry from each. This is the Axiom of Choice.