Denis Bonnay (Paris) gives a talk at the MCMP Colloquium (8 Dec, 2011) titled "Constants and Consequences (joint work with D. Westerstahl)". Abstract: Given an interpreted language and a set of logical constants, Tarski's semantic definition of logical consequence yields a consequence relation. But given a consequence relation, is there a natural way to extract from it a set of logical constants? In this talk, we will compare two ways of doing so, one purely syntactical, which is based on the idea that an expression is logical if it is essential to the validity of at least one inference, and one semantical, which is based on the idea that an expression is logical if its interpretation is fully determined by the rules for its use.