A paradox of volcanology is that the most fundamental part of its knowledge – the observation of volcanic processes and objects – is less strict and less well organized compared with experimental and theoretical approaches determined by these observations. The object of this study is the knowledge resulting from volcanological observations, and the objective is to give this knowledge strict form. For this, we elaborate the methodology of assessment of observational knowledge by means of propositional logic. Two kinds of assessments are developed: (1) assessment of individual statements and rationales for truth and satisfiability; and (2) assessment of a set of statements for self-consistency and deducibility of a statement from the set. Propositional logic allows an analyst to find controversies and contradictions, build self-consistent domains of observational knowledge, and obtain strict inference within these domains by means of logical calculi. We believe that the results of reported work can be used in field volcanological studies for optimization of data interpretation, in hazard assessment for evaluation of recommendations of experts and in risk mitigation for improving communication between scientists and non-professionals.