The purpose of this essay is to present and analyse the basic assumptions of Leszek Nowak’s conception of the unity of science. According to Nowak, the unity of science is manifested in the common application of the method of idealisation in scientific research. In accordance with his conception, regardless of the discipline they represent, researchers go through the same stages in building a theory. Two key ones among them are: introducing idealising assumptions into the representation and then their concretisation. In (...) this view, idealisation is the basis of the scientific method, while other cognitive procedures complement it. Nowak’s conception has particular relevance in the context of the dispute between naturalism and anti-naturalism and in the context of the continuing rift between social scientists and natural scientists. It calls into question the anti-naturalist thesis of the ontological uniqueness of social sciences and the resulting methodological consequences. I argue that Nowak’s conception is a cognitively valuable contribution to the contemporary epistemology of science, but it also has weaknesses, mainly due to the limitations of applying the idealisation-concretisation scheme in research practice. For it turns out, as I point out in this essay, that many idealising assumptions are not subject to concretisation and that concretisations do not always condition an increase in the explanatory and/or predictive power of the representations. (shrink)