Although sustainable development is a late twentieth-century construct, its foundations can be traced to classical legal cultures, particularly Roman and Jewish law, which embedded resource stewardship, communal access, and intergenerational equity. Contemporary legal discourse presents numerous definitions; this study therefore conducts a selective comparative analysis, emphasizing the Hungarian legislative definition and its environmental focus. The research demonstrates that a precise and unambiguous definition of sustainable development is essential, as the pathway toward sustainability remains unclear without it. It is not enough to ensure “the long-term” improvement of quality of life and preservation of biodiversity, it is crucial to prevent the collapse of ecological systems. In the context of sustainability, the temporal dimension emerges as paramount, necessitating a perspective “sub specie aeternitatis” – that is, an orientation toward the perpetual existence of humankind, framed within the horizon of eternity. Although the tendencies appear unfavorable, or even hopeless, there remains potential for a positive outcome.