In the nineteenth century, the concept of decadence was not solely of aesthetic interest but had a number of scientific applications. Decadence itself is an organic metaphor, extending the natural processes of decline and decay to societies and the arts. Rather than rejecting nature outright, decadent authors readily embraced new scientific theories that changed the way people thought about the natural world. The pessimism of nineteenth-century science stemmed from the brutal world of industrial capitalism in which it was developed. Decadent writers then incorporated both scientific ideas and language into a literary style obsessed with decay and decline. Finally, science returned to decadent literature to pathologize certain modes of artistic expression as yet another sign that certain types of individuals were ‘degenerate’. Three key scientific theories of the nineteenth century underpin the decadent fixation on decline, decay, and degeneration: uniformitarianism, evolution, and the conservation of energy. All three theories identify impermanence in natural structures previously believed to be permanent and stable.