In fact, sustainable development, in some of its earlier iterations, was and in some cases is still depicted as three-legged stool, i.e., with the environment, the economy, and society as the legs. This model treats each of the three pillars as separate and equal entities.

The underlying conceptualization of the stool is that if any leg is less important (shorter) or missing the stool will be unstable. However, if all three legs are the same length (each pillar being given equal weight), the result will be a balanced stool that will support sustainable development. This is sometimes referred to as the "Three E's balance rule": Environment, Equity, Economy (SOGESID, n.d.).

Dawe and Ryan (2003) report finding over 4000 uses of the pillars/stool analogy for sustainable development. However, they also find this depiction to be conceptually flawed. Their complaint is that this is not a question of balance. They argue that humanity cannot have either an economy or social well-being without the environment being considered as different and more significant level than either the economy or social well-being.

What is wrong with this model? Simply put, humanity can have neither an economy nor social well-being without the environment. Thus, the environment is not and cannot be a leg of the sustainable development stool. It is the floor upon which the stool, or any sustainable development model, must stand. It is the foundation of any economy and social well-being that humanity is fortunate enough to achieve (p. 1459).

Although the model suggests that all three pillars must be addressed to achieve sustainable development, in fact the weight and attention given to each pillar all too often reflects the orientation of an interest group's agenda (Wironen, 2007). Not surprisingly there is a great deal of criticism of this rather un-dynamic model, and let us acknowledge that there are good reasons for this. According to Lehtonen (2004), it is reasonable to conclude that the three dimensions of sustainable development are not qualitatively equal, but instead occupy different positions in a hierarchy. In line with this critique of the three-legged stool metaphor, others have adopted more complex and integrated models, such as the "egg" and/or Venn diagrams. In Figure 2 (Wironen, 2007) we see an egg model that would respond to the Dawe and Ryan critique, with the environment surrounding society and economy. In Figure 3 we see a Venn diagram showing the overlap between the three elements.

While the egg metaphor and Venn diagram may be a more complex and dynamic models than that of the three-legged stool of sustainable development, for other scholars these still fall short of capturing the essential features of sustainable development because they commonly omit one or more key elements. Beyond the original mainstream notions of sustainable development based on the tri-dimensional concept featuring the interface between environment, economic and social sustainability - subsequent discourse has opened up further reflection on emerging issues. In this discourse two key elements that have been identified as missing from the original tri-dimensional conceptualization are the institutional dimension and the cultural dimension.