In his passionate and moving talk, Leonardo Boff raises the crucial problem of how to devolop a collective perception of enviromental risk and an ecological ethics in the public sphere. It is the problem known in the social sciences, after G. Hardin’s phrase, as “the tragedy of the commons”. Why doesn’t people usually engage in protecting common goods and resources? This paper is concerned with cultural anthropology’s view of the problem. I argue, on the one side, that the usual formulation of the “tragedy of the commons” is biased by ethnocentric and utiliatarian prejudices; on the other side, that ethnography of “altruistic” practices may help to undertand the conditions of a virtuous public morality. As an example, I discuss the institution of blood donation, stressing the link between “social altruism” and local associative relations rooted in the so-called “civil society”.