The paper aspires to provide an (intended new) perspective on the important concept of sustainability rationality and sustainable path/chreode of the societal system. To this end, the concept of social entropy is convoked and examined, so that the social entropy mark can work as an (ordinal) predictor for the sustainability property of the society. The research uses the logical approach, so the discussion is carried on in its most general and abstract dimensions, to provide (possible) solutions for the most different particular historical or geo-political locations. Some new approaches are presented regarding the societal system, social entropy, the very concept of sustainability, and the quantification (including a log-function) of the social entropy mark which, in turn, quantifies the sustainability state. The rapport between the capitalist model of rationality (currently working through the optimality paradigm) and sustainability is examined and concluded, as well as the rapport between sustainability and ethics—the last being, today, an issue intensively debated and un-concluded yet.