This philosophical reflection explores the concept of reactivity as the absolute fundamental and defining principle governing the entire universe. The author operates on the premise that any entity existing within this universe must inherently possess the capacity to react to its environment, as anything independent of reactivity could not meaningfully enter or exist within our reality. The text delves deeply into distinguishing between processes that occur independently of a specific subject and those events that require its direct mediation. A central point of the analysis is the relationship between the subject and the object of reaction, specifically looking at the extent to which the act of reacting influences that which is being reacted upon. While the impact of a single, minute subject might appear negligible, the text argues that in the broader context of mass reactivity, no element is truly zero or insignificant. Special attention is given to the phenomenon of collective reactivity, where the interplay and mutual influence of numerous similar subjects create a complex systemic dynamics.