There is "no established convention" for defining "developing country". According to economist and sustainable development expert Jeffrey Sachs, the current divide between the developed and developing world is largely a phenomenon of the 20th century. The late global health expert Hans Rosling has argued against the terms, calling the concept "outdated" since the terms are used under the prerequisite that the world is divided in rich and poor countries, while the fact is that the vast majority of countries are middle-income. Given the lack of a clear definition, sustainability expert Mathis Wackernagel and founder of Global Footprint Network, emphasizes that the binary labeling of countries is "neither descriptive nor explanatory". Wackernagel and Rosling both argue that in reality, there are not two types of countries, but over 200 different countries, all faced with the same laws of nature, yet each with unique features.