This chapter assesses the claim of science to be knowledge of the real world and examines the notions of objectivity as attributable to science and of reality as imputed to the world it studies. Aristotle's theories together formed a body of doctrine which most writers would describe as metaphysical. But few would deny that at the same time they included theories rightly described as scientific, and that they inspired a vast amount of scientific and quasi-scientific research as well as some very fruitful scientific speculations. The enunciation of a theoretical system, like the Aristotelian, the Newtonian or the Einsteinian, is in the first instance largely schematic and abstract, and the period of Kuhnian normal science is mainly one of application and the development of implications inherent in such application. Chemists have made of phlogiston a vague principle which is never rigorously defined, and which, in consequence, is adapted to every explanation into which it is introduced.