Emergence has a long and controversial history. In this paper we briefly review the primary strands of the debate, paying attention to its use in the fields of philosophy of science and mind, social science and systems theory including the theory of complex systems. We argue that it is important to recognize why emergence in social systems is fundamentally different from other natural systems. The key characteristics of reflexivity are discussed and a distinction between two classes of emergence proposed. Non-reflexive emergence: where the agents in the system under study are not self-aware, and Reflexive emergence: where the agents in the system under study are self-aware and linguistically capable. We specify the generative processes we believe are associated with each of these categories and argue for the adoption of this distinction in both theoretical and practical modeling of human social systems.