What does it mean to be Reactive? The concept of Reactive Programming has gained much traction in the last few years as a paradigm well-suited for the development of asynchronous event-driven applications. Unfortunately, Reactive Programming has been at the center of much discussion, if not confusion, with regards to its definition, properties and identifying principles. In this work we are going to wield the most powerful tool available to software engineers, mathematics, in order to formally derive the reactive types and bring clarity to this much opinionated topic.