A reasonable system

Reasonable is an adjective we typically apply to humans, or something we implore of them (“Be reasonable!”). And, while I do want reasonable colleagues, what I really want is a reasonable system.

By reasonable system, I mean a system whose behavior I can reason about, both backwards and forwards in time. Given my understanding of how the system works, and the signals that are emitted by the system, I want to be able to understand its past behavior, and predict what its behavior is going to be in the future.