In this chapter, I will outline the bestknown and most promising forms of objective realism (OR) in philosophy of science and explain how they differ from each another in terms of their arguments for OR. The first subchapter introduces the most important current of OR in philosophy of science, naturalism in its various forms. From the debate surrounding this term, I will argue for a definition of a realistic naturalism that must be categorised as OR in the context of philosophy of science (4.1.1). I go on to show that this definition of naturalism is valid and not a straw man. To this end, I analyse a selection of the central ideas of one of the most important naturalist of the 20th century, Quine (4.1.2). These include the idea of ontological commitments acquired through the acceptance of scientific theories, of the unity of science, and of reductive physicalism as constituting core features of what I call reductive naturalism. My aim is not to present a comprehensive account of Quine’s position on science, but to show that he advocates a naturalism as encompassed by my definition and to clarify the over-all form he gives it through the additional theses of the unity of science and reducibility to physics. The persistent lack of empirical unity in the natural sciences and the problems of reduction and theories of supervenience are addressed in the following step (4.1.3). They motivate the search for alternatives within objective realism and naturalism. Two influential variants of objective realism in philosophy of science are reconstructed in the second subchapter. First, I consider a proposal for how naturalism can be advocated without reductionism. To this end, I will analyse the naturalist work of Wilfrid Sellars, who, like Quine, is still extremely influential in today’s debate (4.2.1). Here, too, my aim is not to develop a full-blown interpretation of Sellars, but to demonstrate his approach to a non-reductive naturalism in philosophy of science, which, as with Quine’s naturalism, still finds many adherents today. By providing a glimpse into the interpretative debate surrounding Sellars, I strive to show that different readings of Sellars fashion different varieties of OR in the metalocal debate in philosophy of science. By looking at the scope for interpretation, it also becomes clear that a lucid philosophy of science is needed in order to introduce non-reductive naturalism as an alternative to reductive naturalism. To this end, I go on to discuss Anjan Chakravartty’s attempt to clarify the question what kind of metaphysics any realism in philosophy of science must and should presuppose (4.2.2). This opens a route towards Chakravartty’s minimal position of realism, his so-called semirealism. Here, too, Chakravartty’s position will not be comprehensively reconstructed. Instead, I show that Chakravartty’s take leads to a lucid non-reductive naturalism and therefore counts as a promising OR in the context philosophy of science. This step also shows that naturalism is still the standard form of OR in philosophy of science today. In conclusion, the reconstructions and discussions will allow me to specify the hypothesis underlying philosophy of science’s OR in today’s debate. This prepares the ground on which, in the concluding chapter (5) of Part I, I can prove my hypothesis of the double stalemate on the basis of the most important arguments concerning OR in philosophy oh science.