De Witt contributed to financial mathematics: "The Worth of Life Annuities Compared to Redemption Bonds". This work combined his roles as statesman and as mathematician, and was discussed in the correspondence between Leibniz and Bernoulli concerning the use of probabilities. Ever since the Middle Ages, a life annuity was a way to obtain a regular income from a reliable source. The state, for instance, could provide a widow with a regular income until her death, in exchange for a 'lump sum' up front. There were also redemption bonds that were more like a regular state loan. De Witt showed that for the same principal a bond paying 4% interest would result in the same profit as a life annuity of 6% (1 in 17). But the 'Staten' at the time were paying over 7% (1 in 14). The publication about life annuities is "one of the first applications of probability in economics." After the violent deaths of the brothers the 'Staten' issued new life annuities in 1673 for the old rate of 1 in 14.