We think we know Lambert Strether. Henry James names the unlikely hero of The Ambassadors (1903) after Honoré de Balzac's unlikely novel Louis Lambert (1832–33)—or so he says. In this essay I argue that James's choice is also influenced by his knowledge of the eighteenth-century Alsace philosopher J. H. Lambert. Now obscure, Lambert was, in his day and in the nineteenth century, a major figure in European science and philosophy, one deeply influential on the formation of phenomenology and pragmatism, disciplines closely associated with James's brother William James. Lambert's fascination with the problem of appearances also offers connections with Strether's experience in Paris and invites an exploration of the role of visual art in James's novel, including Hans Holbein's masterpiece with which it shares a name. In this study I argue that the name of Lambert, far from offering an easy clue to Strether's identity, offers him a variety of possible natures and possible ways of viewing reality.