Message passing
Computation as independent objects sending messages to each other — the core of Alan Kay's idea of objects.
For Alan Kay, the heart of object-oriented programming was never classes or inheritance — it was messaging. An object is like a tiny computer on a network: it has its own state, it keeps that state private, and it interacts with the world only by sending and receiving messages.
Kay's inspiration was partly biological (cells communicating chemically) and partly the early ARPANET. He later wrote that he regretted coining the term "object-oriented," because it put the focus on objects rather than on messages: "The big idea is 'messaging.'"
The consequence is profound. If objects only ever talk through messages, then what an object does in response to a message can be decided as late as possible — see late binding — and systems can grow and change while running, the way the internet does. This is the vision Smalltalk tried to make real.