Trustworthiness by default Johan W. Klüwer 1 and Arild Waaler 2 1 Dep. of Philosophy, University of Oslo johanw@filosofi.uio.no 2 Finnmark College and Dep. of Informatics, University of Oslo, arild@ifi.uio.no. But never put a person to death on the testimony of only one witness. There must always be at least two or three witnesses. Deuteronomy 17:6 (New Living Translation) Abstract. We present a framework for reasoning about trustworthiness, with application to conflict resolution and belief formation at various de- grees of reliability. On the basis of an assignment of relative trustworthi- ness to sets of information sources, a lattice of degrees of trustworthiness is constructed; from this, a priority structure is derived and applied to the problem of forming the right opinion in the presence of possibly con- flicting information. Consolidated with an unquestioned knowledge base, this provides an unambiguous account of what an agent should believe, conditionally on which information sources are trusted. Applications in multi-agent doxastic logic are sketched. 1 Introduction To trust an information source, in the simplest, unconditional form, is to be- lieve every piece of information that the source provides. While providing a paradigm, this notion of trust has limited application to realistic scenarios. In general, the trust we have in our information sources, which may vary in kind from teachers to newspapers to legal witnesses, is not unconditional: we believe what we are told by a trusted source only as long as we don’t possess knowledge to the contrary. This simple observation motivates the approach to trust that we will be discussing in this paper. Conditional trust in an information source is a default attitude: To believe what you are told, unless you know better. When looking for information, we often need to consider several sources. Sources may vary widely with regard to their reliability, and a cautious default approach then informs us to let the more trustworthy ones take priority over those that are less trustworthy. Furthermore, we often need to consider more than one source at a time. Notions of agreement or corroboration, as well as the consolidation of information drawn from different sources, are essential. What we present here is a framework for reasoning about relative trustwor- thiness, with sets of information sources as the basic trusted units. The main part of the paper is structured as follows. Section 2 addresses properties of the trust relation itself, making only informal reference to notions of information. 1/15