The One-to-One Principle says that conceptually disinct entities, such as a painting and a digital image of the painting, should be described by conceptually distinct descriptions. The principle was formulated in the early years of Dublin Core in order to draw attention to, and challenge, the widespread practice of creating metadata that pragmatically conflated elements descriptive of conceptually distinct resources into a single record.
For example, a metadata record about a book might include properties of the author (i.e., dates of birth and death, affiliation), or a metadata record about a painting (e.g., "Mona Lisa") might include a properties of the photograph itself (a JPEG image). Humans can be quite good at disambiguating such descriptions, but not machines. Accepting RDF as the basis for metadata interoperability allowed Dublin Core descriptions both to respect the One-to-One Principle and to transcend the limitations of "flat" single-resource descriptions.