Andreas Barrels (Bonn) gives a talk at the MCMP Colloquium (7 May, 2014) titled "How to Bite the Bullet of Quidditism - Why a Standard Argument against Categoricalism in Physics Fails". Abstract: Categoricalism is the statement that fundamental properties of physics are categorical, i.e., they have their dispositional characters not with metaphysical necessity. According to Black (2000), Bird (2005, 2007), and Esfeld (2009), categoricalism entails quidditism, the possible existence of properties which are not exclusively individuated by their dispositional characters. If quidditism is true, we cannot know, in principle, whether it is property F or its “Doppelgänger” G that shows up by exhibiting a certain set of dispositional characters. Since we cannot accept our metaphysics of properties to condemn us to necessary ignorance of fundamental properties, we must reject quidditism. Therefore, categoricalism fails. I argue that the possible epistemic situation revealed by quidditism is a case of empirical underdetermination of theoretical properties. This type of situation is not conceived, in general, as the occurrence of some necessary limit of knowledge. There are rational procedures to deal with empirical underdetermination in physics, and thus to decide about the properties the existence of which we are committed to accept. Thus, the unacceptability claim against quidditism is not well founded and categoricalism cannot be defeated that way.