We would all agree that there are unexpected events in the world, and that many of them occur at exactly the same time. It is also clear that the state of any given system may be composed of any number of sub-states, where the full consequence of even minor state changes are difficult to predict—the power of a butterfly's wings being enough to tip a much larger system into an alternate state. And we also know that the volume and shape of a system, over time, changes in ways difficult to predict.
In his PHD thesis "Foundations of Actor Semantics", written in 1981, William Clinger proposed that his work was:
...motivated by the prospect of highly parallel computing machines consisting of dozens, hundreds or even thousands of independent microprocessors, each with its own local memory and communications processor, communicating via a high-performance communications network.
As it turns out, Clinger was on to something. Concurrency is a property of systems composed of...