This paper seeks an explanation for the challenges faced by Semantic Web developers in achieving their vision, compared to the staggering near-instantaneous success of the World Wide Web. To this end it contrasts two broad philosophical understandings of meaning and argues that the choice between them carries real consequences for how developers attempt to engineer the Semantic Web. The first is Rene Descartes' "private," static account of meaning (arguably dominant for the last four-hundred years in Western thought), which understands the meanings of signs as whatever their producers intend them to mean. The second is Charles Peirce's still relatively unknown " public," evolutionary account of meaning, according to which the meaning of signs just is the way they are interpreted and used to produce further signs. It is argued that only the latter approach can avoid the unmanageable attempts to "preprocess" interpretation of signs on the Web that have dogged the project in its many stages, and thereby do justice to the scale, rapid changeability, and exciting possibilities of online information today.