 In these debates, the notion of levels of organization is typically used as a primitive term that is assumed to be clear enough and is therefore left undefined (Griesemer 2005). A notable exception is Okasha (2006), who puts forward a proposal for understanding levels of organization in natural selection, building on earlier work by McShea (2001). The starting point is the part-whole relationship, which is the standard definitive feature of levels of organization, but taken alone is insufficient for defining levels of organization: A big heap of sand is made up of smaller heaps of sands, but does not constitute a higher level of organization. Therefore, Okasha (2006) and McShea (2001) propose two further conditions: The parts that form higher-level wholes must interact with each other, and they must be homologous with organisms in a free-living state. For example, the cells that compose organisms interact with each other and are homologous to free-living unicellular organisms, and therefore constitute a level of organization. On the other hand, candidates such as heaps of sand are ruled out, as they do not significantly interact with each other, nor are they homologous to free-living organisms.