This paper suggests that the failure to integrate history and philosophy of science properly may be explained by incompatible metaphysics implied by these fields. Historians and sociologists tend to be historicists, who assume that all objects of research are variable in principle, while philosophers look for permanent and essential qualities. I analyse, how the historicists and essentialist approaches differ with regard to the research objects of general history, history of science and science itself. The implied historicism makes some radical pronouncements by Latour on ontological variance understandable. I will also consider, whether there could be something like a historicist philosophy of science. The historicisation of the natural world proves most challenging, but both certain traditional disciplines and some recent advances in physical and life sciences indicate compatibility with historicism. One should note that historicism does not alter how ‘truth’ is understood. Historicism does not question the reality of objects either; only their eternality.