We start from the claim that trust in information sources is just a kind of social trust. We are interested in the fact that the relevance and the trustworthiness of the information acquired by an agent X from a given number of sources strictly depends and derives from the X's trust on each of these sources with respect the kind of that information. In this paper, we analyze the different dimensions of trust in information sources and formalize the degree of subjective certainty or strength of the X's belief P, considering three main factors: the X's trust about P just depending from the X's judgment of the source's competence and reliability; the sources' degree of certainty about P; and the X's degree of trust that P derives from that given source. Finally we present a computational approach based on fuzzy sets.
