What is bias? A review of the library literature reveals no attempts to define the concept. Nor does it reveal systematic attempts to develop interventions that teach the identification and evaluation of bias. Current pedagogical approaches (checklists and bias charts) tend to assume a self-evident definition that categorises bias as unquestioningly bad and disqualifying. Current approaches, however, fail to recognise the cognitive complexity of decoding bias within a source. A decoding process includes identifying the type of bias, determining an objective baseline, recognising biased features, and analysing bias's impact. Based on work done from several fields--argumentation theory, media bias, media literacy, and history education--this paper proposes an operational definition of bias and a practical framework for conceptualising a process to identify and evaluate bias. This paper will explore the limitations of this framework, as well as existing source evaluation paradigms. If librarians want to prepare individuals to participate in a post-truth society, where disinformation weaponises bias by appealing to emotions and beliefs rather than facts, an inclusive and nuanced conception of bias is a necessary component of library instruction.