Newton's 3rd Law may not apply exactly to workflows, but the marketing people here love the verbiage. So we can probably better describe the "workflow Law" as every Trigger Event shall execute an Action Event.
A Trigger Event can best be described as an event that our system waiting and watching to happen, such as receiving a text or a form submission.
Action Events are the events that are executed by a trigger event, like sending an email (action) after a receiving a form submission (trigger).