# What's Inside the Arrows?

Inside black boxes you find programs. There are a lot of programming languages. You may not care how black box $$A$$ works as a substitute for black box $$B$$ if it’s observationally equivalent to black box $$B$$.

How do the boxes interact? What’s inside the arrows that connect the boxes? Inside arrows you find protocols. There are a lot of protocols.

Are you looking in the wrong place to see what’s going on? In practice, effort to understand e.g. English – the messages we send back and forth – is more fruitful and useful than effort to understand what’s going on in the brains of a system of people.1

1. Joe Armstrong, “The How and Why of Fitting Things Together”, Erlang Factory Conference (2013). ↩︎