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Sustainable finance integrates, to one degree or another, environmental, social and governance considerations into investment decisions. It works for the common good but faces a theoretical gap: the foundational concept of common utility makes no economic sense. By default, common good relates to extra-financial reporting, not-sophisticated investors and outside banking channels trading (such as crowdfunding). The intent of the paper is, from a behavioral economics perspective, to model the reasoning and decision behaviors of both sophisticated and not-sophisticated investors, while common utility measures the satisfaction (vs dissatisfaction) or welfare (vs discomfort) that the greatest number finds in the consumption or the obtaining of the common good by a small number. Common utility can therefore be understood relatively, as an original utility at stake at a time when it ensured the survival of mankind and when property had not yet begun to threaten the common good and cause its downfall (Tragedy of the commons). Its coupling with Markowitz’s modern portfolio theory gives a postmodern version of it, at the service of sustainable finance and with experiments in sustainable entrepreneurship.