A term used in science, systems theory, philosophy, urbanism and even art, "emergent properties" or "emergence" refer to those properties that arise from the collaborative functioning of a system, but do not belong to any one part of that system. In other words, emergent properties are properties of a group that are not possible when any of the individual elements of that group act alone. Cities, the brain, ant colonies and complex chemical systems, for instance, all exhibit emergent properties that serve to illustrate the concept.
A single ant is a rather limited organism, with little ability to reason or accomplish complex tasks. As a whole, however, an ant colony accomplishes astounding tasks, from building hills and dams to finding and moving huge amounts of food. In this context, emergent properties refer to the qualitative changes that occur in ant behavior when individual ants work together. Alone, an ant behaves erratically and almost at random, but the sum of millions of random actions serves to identify necessary tasks and organize other ants to complete them. An ant that finds food, for example, secretes a small amount of a hormonal substance that attracts other ants which, in turn, also secrete that same substance when they reach the same food source. Thus, millions of wandering ants become the organized straight lines leading to the nearest picnic. The organization of ants, only possible when the system works as a whole and individual actions reinforce each other, is an emergent property.
Human consciousness is often called an emergent property of the human brain. Like the ants that make up a colony, no single neuron holds complex information like self-awareness, hope or pride. Nonetheless, the sum of all neurons in the nervous system generate complex human emotions like fear and joy, none of which can be attributed to a single neuron. Although the human brain is not yet understood enough to identify the mechanism by which emergence functions, most neurobiologists agree that complex interconnections among the parts give rise to qualities that belong only to the whole.
Chemistry studies a number of cases where individual forces or actions do not necessarily add up to a simple sum of the parts. In physics, two forces acting on one body naturally increase the total force, yet chemistry is concerned with cases where, due to complex organizations of atomic energy in elements and compounds, certain chemical reactions create entirely new elements, compounds or sources of energy that are not a simple combination of the effects of the parts involved. Neutralization reactions, for example, were used by the philosopher John S. Mill to describe situations where cause-and-effect principles for each of the parts involved in a reaction could not predict the outcome. When hydrochloric acid and sodium hydroxide combine, the result is salt and water, a product not at all consistent with the effects of either a strong acidic or basic compound.
The complex social organization of humans also exhibits certain emergent properties. Social scientists and urban planners often point to cities as the clearest example of emergence in human interaction. They study how certain areas of a city tend to develop similar economic or social activities and gradually become specialized hubs from theater districts to large fish markets. Especially in the case of activities that are not controlled by zoning regulations, the decision of one individual to conduct a certain activity in a certain place tends to make similar or complementary activities in the vicinity more feasible. If one person opens a theatre on a street, the area begins to be frequented by people looking for cultural activities, until the street attracts art galleries and schools and gradually becomes a cultural district. No single person makes the decision to generate a cultural center, but the confluence of interests creates the space through emergent properties.