This article undertakes the anthropology of an equation that constitutes the essence of an algorithm that underlies a variety of computational technologies—most notably spam filters, but also data-mining tools, diagnostic tests, predictive parsers, risk assessment techniques, and Bayesian reasoning more generally. The article foregrounds the ways ontologies are both embodied in and transformed by such algorithms. And it shows the stakes such ontological transformations have for one particularly widespread and powerful metaphor and device—the sieve. In so doing, this inquiry shows some of the complex processes that must be considered if we are to understand some of the key relations linking semiosis and statistics. Reflexively, these processes perturb some core ontological assumptions in anthropology, science and technology studies, and critical theory.