The purpose of this book is to provide a sound economic foundation for finance. Finance is a coherent branch of applied economics that aims to understand financial markets in order to offer advice for practical financial decisions. This book argues that for a solid economic foundation in finance, the famous general equilibrium model, which in its modern form emphasizes the incompleteness of financial markets, is well suited. The aim of the book is to demonstrate that financial markets can be meaningfully integrated into a broader system of markets, including, for example, commodity markets. The interaction of these markets can be described using the well-known concept of competitive equilibrium. We argue that for a sound foundation, this competitive equilibrium should be unique. In the first step, we show that this essential goal cannot be achieved based solely on the rationality principle.