Biological organisation, or the hierarchy of life, is the hierarchy of complex biological structures and systems that define life using a reductionistic approach. The traditional hierarchy, as detailed below, extends from atoms to biospheres. The higher levels of this scheme are often referred to as ecological organisation. Each level in the hierarchy represents an increase in organisational complexity, with each "object" being primarily composed of the previous level's basic unit. The basic principle behind the organisation is the concept of emergencethe properties and functions found at a hierarchical level are not present and irrelevant at the lower levels. Organisation furthermore refers to the high degree of order of an organism. Ideally, individual organisms of the same species have the same arrangement of the same structures. For example, the typical human has a torso with two legs at the bottom and two arms on the sides and a head on top. It is extremely rare to find a human that has all of these structures but in a different arrangement.