This paper analyzes the relationship between monetary policy and financial stability in the Banking Union. There is no uniform global model regarding the relationship between monetary policy-making on the one hand, and prudential supervision on the other. Before the crisis, EU Member States followed different approaches, some of them uniting monetary and supervisory functions in one institution, others assigning them to different, neatly separated institutions. The financial crisis has underlined that monetary policy and prudential supervision deeply affect each other, especially in case of systemic events. Even in normal times, monetary and supervisory decisions might conflict with each other. After the crisis, some jurisdictions have moved towards a more holistic approach under which monetary policy takes supervisory considerations into account, while supervisory decisions pay due regard to monetary policy. The Banking Union puts prudential supervision in the hands of the European Central Bank (ECB), the institution responsible for monetary policy. Nevertheless, at its establishment there was the political understanding that the ECB should follow a policy of meticulous separation in the discharge of its different functions. This raises the question whether the ECB may pursue a holistic approach to monetary policy and supervisory decision-making, respectively. On the basis of a purposive reading of the monetary policy mandate and the SSM Regulation, the paper answers this question in the affirmative. Effective monetary policy (or supervision) requires financial stability (or smooth monetary policy transmission). Moreover, without a holistic approach, the SSM Regulation is more likely to provoke the adoption of mutually defeating decisions by the Governing Board. The reputation of the ECB would suffer considerably under such a situation - in a field where reputation is of paramount importance for effective policy. As any meticulous separation between monetary and supervisory functions turns out to be infeasible, the paper explores the reasons. Parting from Katharina Pistor's legal theory of finance, which puts the emphasis on exogenous factors to explain the (non)enforcement of legal rules, the paper suggests a legal instability theorem which focuses on endogenous reasons, such as law's indeterminacy, contextuality, and responsiveness to democratic deliberation. This raises the question whether the holistic approach would be democratically legitimate under the current framework of the ESCB. The idea of technocratic legitimacy that exempts the ECB from representative structures is effectively called into question by the legal instability theorem. This does not imply that the independence of the ECB should be given up, as there are no viable alternatives to protect monetary policy against the time inconsistency problem. Rather, any solution might benefit from recognizing the ECB in its mixed technocratic and political shape as a centerpiece of European integration and improving its transparency, responsiveness, and representativeness without removing its technocratic character.
