Is a general theory of memory possible? What contribution can sociology make to this vast scientific project? Two original contributions are presented and discussed in this article: (1) the concept of collective memory (Maurice Halbwachs); (2) the concept of social memory (Niklas Luhmann). The author proposes some important theoretical clarifications. First, memory is neither a stock nor a collection (of past states or events) but rather a continuous operation of sorting between remembering and forgetting. Secondly, collective memory is not social memory: indeed, the former is a specific operation of psychic systems whereas the latter is an operation of communication, specific to social systems. In the particular case of the attacks of Nove...