Neil Smith's writings about capitalism and what we call “nature” were insightful and influential. This paper asks what Smith would make of the “radical turn” today occurring in the world of international geoscience. If we “think with” Smith, how should we view Naomi Klein's recent statement that geoscientists can act as fifth columnists calling the capitalist way of life into question? In the first half of the essay I address these questions. I summarise and apply the insights of Smith's writings to recent developments in international geoscience. Smith wrote about science in most of his published statements about capitalist ecology and I show that he would ultimately have regarded Klein as hopeful, even naïve. I then go on, in the second half of the essay, to “think against” Smith. I suggest his views on science bespeak a wider, unhelpful separation between Left scholarship in the social sciences and humanities and the STEM subjects (science, technology, engineering, and medicine). Recalling earlier attempts to radicalise science politically, and highlighting the radical potentials of geoscience today, I make the case for forms of interdisciplinarity that might render geoscience more political. Though this case opens space for perspectives beyond the Marxism Smith did so much to develop, he would—I hope—see it as a legitimate part of the Left's long war against capitalism's rule over society and environment.