The idea of a 'theory of everything' is inconsistent with a natural feature of biological evolution: the spontaneous emergence of composite entities with completely new properties. At successively higher levels of complexity, from elementary particles and chemical molecules, through unicellular and multicellular organisms, to self-aware human beings and their cultural institutions, we find systems obeying entirely novel principles. The behaviour of such systems is not predictable from the properties of their constituents, so distinct 'languages' are required to describe them scientifically. The plurality of our sciences is thus an irreducible feature of the universe we live in. In particular, the reversible time coordinate of mathematical physics cannot cope with the natural 'path dependence' of biology. In the human sciences this extends into the imagined future as well as the remembered past. Furthermore, science nowadays usually arises in localized social contexts, where the 'logic of the situation' is continually being transformed by the emergence of cultural novelties such as unpredictable technological innovations. Thus, scientific knowledge cannot be restricted to generalized synchronic models, but involves historical narratives of specific events and unforeseen circumstances.