All properties of a complex system that are not relations between it and something else derive from the properties of its constituents and their effects on each other when so combined. Emergence is an epistemological condition: it means that an observed feature of the system cannot be derived from the properties currently attributed to its constituents. But this is a reason to conclude that either the system has further constituents of which we are not yet aware, or the constituents of which we are aware have further properties that we have not yet discovered. Thomas Nagel.
