Cost-benefit analysis (CBA) is often regarded as a pure form of practical reasoning that can shift accountability onto supposedly impersonal mechanisms, summarize complex choices in a formulaic way, and transmute popular pressure, political debate and political conflict into quiet, office-bound operations performed on fixed and agreed-upon preferences. Yet CBA’s commensuration of things that no one has any experience in commensurating leads to odd new ways of treating reason, democracy, public opinion, space, time and personhood. And the more practical steps are taken toward its algorithmic ideal of decision-making, the more unforeseen political and social difficulties crop up, including popular resistance. For other work on cost-benefit analysis see http://www.thecornerhouse.org.uk/resource/whose-voice-speaking and http://www.thecornerhouse.org.uk/resource/toward-different-debate-environmental-accounting.