The study critically examines the concept of sustainability by focusing on the arguments of Derek Parfit and Hitoshi Nagai. The concept of sustainability is based on that of intergenerational ethics, which asserts the rights of future generations. Parfit's thought experiment "Depletion" shows that we cannot make assumptions about the characteristics of future generations in order to guarantee their rights because of the Non-Identity Problem. Nagai suggests that our ethics are derived from inverted moral and ethical values, and that intergenerational ethics underpinning the concept of Education for Sustainable Development is rooted in the present generational ethics as our inverted values.