You aren't. You don't have a "program." You have millions of tiny isolated processes that can send messages to one another. Since you cannot observe the state o

You aren't. You don't have a "program." You have millions of tiny isolated processes that can send messages to one another. Since you cannot observe the state of any process from the outside, and cannot tell if it has failed or not, you have to rely on other things (think tcp/network protocols) to indicate message reception and processing. You can reason about your protocols, as they have an algorithm. But your object graph as an entire entity is a bunch of black boxes with very few observable properties about which to reason, or indeed even need to in the world of isolation.