This article offers an analysis of Shakespeare’s Othello alongside arithmetic textbooks for merchants and soldiers. It argues that Othello dramatizes a problem that also haunts the pages of these math books: the problem of calculating the value of persons in a society where new forms of commercial credit were unsettling traditional notions of worth grounded in status, military prowess and sexual purity. Othello’s loss of faith in his wife and the disintegration of his sense of self both depend on Iago’s manipulation of two competing models of reputation: one based on martial valor and sexual purity (reputation as honor) and one associated with buying, selling borrowing and lending (reputation as credit). In order to have Desdemona “undo her credit with the Moor,” Iago teaches Othello that reputation is extrinsically constructed, rhetorically grounded, and easily inflated or devalued by means of words. Shakespeare associates Iago with a historically particular, pessimistic view of commercial credit. As an alternative to Iago’s skepticism and to Othello’s more attractive but equally destructive idealism, Othello offers glimpses of a heterogeneous mercantile world where mutual enterprise and cross-cultural encounter produce a mixed, flexible, and sustainable version of social value.