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