The abstraction of physical commodities into quantified financial contracts has played an important role in enabling the mathematisation of western society. In particular, financial ethics has provided the basis on which mathematical probability has been developed. This paper begins by summarizing recent research on the role of commercial ethics in the development of probability. This is related to the Fundamental Theorem of Asset Pricing (FTAP), the foundational theory of contemporary financial mathematics. The paper then connects the FTAP to the Dutch Book Argument (DBA). The paper’s main contribution is in describing the significance of financial practice in justifying the DBA, itself the most popular justification for subjective probability. The paper then explains how markets dynamically confirm beliefs, which is modeled by the FTAP but not adequately by the DBA. The conclusions emphasize the distinction between pure and practical reasoning and that this should be mirrored in a distinction between the mathematics of physical and practical. This point is important as mathematics is becoming more important in modelling, and directing, social systems.