This text presents a critical analysis of the concept of "development" in relation to traditional notions of causality. The author argues that development fundamentally challenges causal thinking by implying a process that is not strictly predetermined from its inception but evolves through external circumstances and internal dynamics. The document suggests a shift in perspective where the emphasis moves from the "action" of causes to selective activities and the capacity of a system to respond to its environment. The author introduces the concept of "reactivity" and proposes a reinterpretation of the "causal nexus" as a "reaction nexus." Rather than seeing development as a passive result of prior causes, it is viewed as a series of active responses that select necessary conditions for their realization. This paradigmatic shift transforms the understanding of phenomena, viewing them not as mere consequences of primary causes but as outcomes of complex interactions and selective processes occurring over time.