 In philosophy, systems theory, science, and art, the emergence occurs when the whole is greater than the sum of the parts, meaning the whole has properties its parts do not have. These properties come about because of interactions among the parts. Emergence plays a central role in theories of integrative levels and of complex systems. For instance, the phenomenon of life as studied in biology is an emergent property of chemistry, and psychological phenomena emerge from the neurobiological phenomena of living things. In philosophy, theories that emphasize emergent properties have been called emergentism. Almost all accounts of emergentism include a form of epistemic or ontological irreducibility to the lower levels. In philosophy, the emergence is often understood to be a claim about the etiology of a system's properties. An emergent property of a system, in this context, is one that is not a property of any component of that system, but is still a feature of the system as a whole. Nikolai Hartmann, one of the first modern philosophers to write on emergence, termed this category will know the new category.