Suppose the latest generation of telescopes detect oxygen in the atmosphere of an exoplanet. Is that a ‘biosignature’? At one time it would have been called a biosignature (by some, at least), but not now. Now we have plausible abiotic stories to tell about the presence of oxygen in the atmosphere of a planet. But what about a case - such as an oxygen-methane disequilibrium - where we currently do not have a plausible abiotic story to tell? Should we call this a biosignature? Or should we expect that in due course we will develop a plausible abiotic story? In this talk I argue that quantifying the uncertainty requires us to quantify the extent to which we have explored the relevant possibility space, a very difficult thing to do. A Bayesian approach to quantifying the relevant uncertainty seems unhelpful.