This article addresses the issue of conventions from the perspective of the early modern guilds’ regulations related to product quality. Starting from the ideas of François Eymard-Duvernay one specific convention is identified: ‘intrinsic value’ (i.e., value related to the raw materials used). This convention enabled guild-based artisans to locate product quality in their political standing and, hence, was intimately linked to the (urban) political context. While this may be familiar to the ideas on ‘justification’ of Boltanski and Thévenot, an historical analysis reveals fundamental conceptual issues. In the seventeenth and eighteenth centuries, both the convention of intrinsic value and the guilds’ power to define product quality became obsolete because of (among other things) epistemological transformations. While intrinsic value as a convention was connected to the idea of matter possessing mysterious, religious and creative powers in itself, natural philosophers naturalized matter from the seventeenth century on. As a result, value may have become synonymous with either the products’ place in a taxonomy of products or the meaning produced in discourses external to the product. Further research should in any case take fundamental epistemic shifts and shifts in the subject-object relationships into consideration.