What is 'is'?

With the abundance of XML vocabularies a common question is: which seemingly different elements are really the same, and which ones are really different. In business we encounter this problem in data exchange: how do I map the messages and elements from my favorite B2B-vocabulary onto the B2B-vocabulary my trading partner uses? Ontologies try to define which things we speak (or exchange data) about and how we reference them. In mapping between two ontologies we often use equivalence relationships: 'LastName = given_name', 'Thomas Mann = der Zauberer' et cetera. In the first part of the paper I want to explore some philosophical notions on equivalence: the difference between intension and extension (Frege) and the idea that meanings aren't always precise (Wittgenstein). I will also discuss the relevance for IT of these notions. In the second part I want to explore some current solutions in XML and Knowledge Management: (1) The naive approach: Let's make a new vocabulary which covers everything, then let everybody use that vocabulary. (2) Adding meta-information: This approach is used in the Context Drivers of ebXML. (3) Published Subject Identifiers (PSI): Make public libraries of unique ID's for things and map to those ID's. In the third part I will identify some problems in the current solutions and propose an enhancement: we need to capture the knowledge in mappings and we need tools to help reusing this knowledge. An open and standardized format for storing and exchanging knowledge about mappings would be a major step towards ontology interoperability.