Maxwell's great paper of 1865 on the theory of the electromagnetic field is analysed in detail, emphasising the revolutionary nature of its contents.Building on his discovery of the equations for the electromagnetic field, he reformulated the theory without recourse to a mechanical model of the field, although his thinking was still strongly influenced by Newtonian mechanics. He emphasised the primacy of fields as the vehicle for transmitting forces and abolished the Newtonian concept of action at a distance. Fields pervaded the aether, endowing empty space with physical properties. In the new formalism, he demonstrated again that light is electromagnetic radiation.