Formalism – the extensive exploitation of mathematics in economic theorising – has been a distinguishing characteristic of economics since the Marginalist Revolution of the 1870s, a process that has intensified through the course of the twentieth century. The application of mathematics to economics has been particularly reflected in the domain of general equilibrium which represented the most fundamental attempt to unify the foundations of the discipline from Walras’s contribution through to its current variant of the Dynamic Stochastic General Equilibrium (DSGE) model, which is at the centre of so much of current orthodox macroeconomics. Meanwhile critics of orthodox economics have stridently criticised what they perceive as the excessive use of mathematics in general in economics, while at the frontiers of research in the discipline, new mathematical approaches have been suggested by a number of pioneering figures who are not opposed to the mathematisation of economics per se, but are opposed to the type of mathematics – classical mathematics in the main – that has been used and continues to be applied in economic theorising. From a methodological point of view, there is an urgent need for economic methodologists and economic theorists in general to direct their attention to a cognate domain of study, namely the philosophy of mathematics particularly its influence on the development of the mathematisation of economics. This is a fundamental informing rationale for this book if our understanding of the genesis, evolution and current developments at the foundational frontier of our discipline are to be methodologically engaged, analysed and understood.