The notion of emergence has accompanied philosophy of science since the late XIX century, claiming that in some systems there are properties in certain levels that cannot be deduced from properties of their components as seen in more fundamental levels. Throughout the XX century, emergence has been characterized by four pillars: unpredictability, novelty, restriction and downward causation. These four pillars have been related to the assumption of a hierarchical order of reality in different levels of organization. In this paper, we (...) show that it is possible to explain the nature of the emergent properties through the nearly-decomposability criterion introduced by Herbert Simon. (shrink)