We present a formalisation for the notion of viewpoint, a construct meant for expressing several varieties of relativised truth. The formalisation consists in a logic which extends first order predicate calculus with its own metalanguage, an axiomatization of probability and proper reflection rules. The extension is not conservative, but consistency is granted. Viewpoints are defined as set of reified meta-level sentences. A proof theory for viewpoints is developed which enables to carry out proofs of statements involving several viewpoints. A semantic account of viewpoints is provided, dealing with issues of self referential theories and paradoxes, and exploiting the notion of contextual entailment. Notions such as beliefs, knowledge, absolute truth or truth relative to a situation can be uniformly modeled as probability in specialised viewpoints, obtained by imposing suitable constraints on viewpoints.