This course of Frege’s reading and lectures during the period of 1874–1879 dovetailed quite naturally with the interests he displayed in his Habilitationsschrift. The ‘extension of the concept of quantity’ referred to in the title concerns the fact that our understanding of quantities (e.g., lengths, surfaces, etc.)  has to be extended in the context of complex numbers.[3] He says, right at the beginning of this work: