This article presents a conventionalist interpretation of the financialization of the economy. We define three periods, each one associated with conventional calculation systems that may shape investment decisions. Each of these periods begins with the adoption by financial practitioners of a new “convention” to make investment decisions: the actuarial convention at the end of the 19th century, the mean-variance convention during the 1970s, and the market-consistent convention since the 1990s. These conventions are rooted in finance theory developments and are associated with different financing circuits for economic activity. When a new convention arises, it does not mean the disappearance of the old one, which can still be used by some practitioners for certain given matters, but it can also redefine some financial professions by fragmenting them according to the convention followed, and it can finally also give rise to new professions.