 Although interest in what can be arguably called formal aspects of deontic logic continued off and on, the trend toward studying logic using the symbolic and exact techniques of mathematics began primarily in the nineteenth century, and became dominant in the twentieth century. Work in twentieth century symbolic modal logic provided the explicit impetus for von Wright (1951a, 1951b), the central early figure in the emergence of deontic logic as a full-fledged branch of symbolic logic in the twentieth century. However, we note that prior to von Wright 1951a, there was one significant earlier episode in symbolic deontic logic, namely Mally 1926. See supplement A:  Mally and Symbolic Deontic Logic.[6]