The Indian librarian and library theorist S.R. Ranganathan (1892-1970) is generally recognized as a seminal figure in the development of facet analysis and its application to classification theory. In recent years, commentators on the epistemology of knowledge organization have claimed that the methods of facet analysis reflect a fundamentally rationalist approach to classification. Yet, for all the interest in the epistemological bases of Ranganathan’s classification theory, little attention has been paid to his theory of how human beings acquire knowledge of the world – i.e., his epistemology proper – or to the question whether this theory reflects a rationalist outlook. This paper examines Ranganathan’s statements on the origins of knowledge to assess if they are congruent with rationalist epistemology. Ranganathan recognized two different modes of knowledge – intellection (i.e., intellectual operations on sense data) and intuition (i.e., direct cognition of things-in-themselves) -- and it is in virtue of the latter that his epistemology can be considered to fall within the ambit of rationalism. Intuition as a source of knowledge plays a role in Ranganathan’s classification theory, most notably in his model of scientific method underlying classification development, his vision of the organization of classification design, and his conceptualization of seminal mnemonics and a reduced number of fundamental categories as important elements in the design of classification notation. Not only does intuition subtend the rationalism of Ranganathan’s epistemology but it also serves as a bridge to another often-neglected aspect of his thought, namely his valorization of mysticism. Indeed, Ranganathan’s theory of knowledge is best characterized as mystical rationalism.