In this column, the author raised the conceptual question of the Semantic Web layer cake's proof layer. He believe that provenance knowledge will be increasingly important at this abstraction level - and possibly the key to establishing trust mechanisms and advanced retrieval methods in decentralized applications. Work on Semantic Web provenance issues has started in parallel in different communities in the last decade. However, it's still far from being complete. Understanding provenance knowledge as a first-class citizen of distributed semantic workflows, knowledge bases, and inferencing and retrieval mechanisms will require further convergence of different viewpoints, standardization for seamless interoperability, and possibly new linking and querying methods for user-oriented navigation in the "Web of provenance".