In this book Georges Ibongu shows that Ernst Cassirer adopts a sort of structural realism in his writings on physics. Cassirer does not deny the existence of physical entities. For him, however, a physical entity loses its absolute fixity and is involved in the process of physical knowledge. The electron is a definite object but cannot, as an individual, be designated by being here and now. The individuality of simple particles means, for Cassirer, that they are only describable as points of intersection of definite relations or structures.