Lecture XII examines the ‘Principles of Classification’, arguing that any true understanding of religion requires a full understanding of the language in which religion and mythology are embodied. Müller’s focus here is on the Aryan and Semitic families of language, although he moves beyond these to other language families (the Ural-Altaic) in Lecture XIII. Müller then poses the question of what language actually is, and attempts to answer this in Lecture XIV, ‘Language and Thought’. As we cannot think, much less communicate, without language, there must be nothing more ancient, or wonderful, than language. Lecture XV, ‘Dynamic Stage’, explores what language can teach us about the origin of religion, and Müller then moves on to a more thorough discussion of the genealogy of myth in the next three lectures (XVI, ‘Mythology’; XVII, ‘The Genealogical School’; and XVIII, ‘The Analogical and Psychological Schools’), demonstrating that myth is a natural and inevitable stage in the development of language and indeed of all thought.

In the final two lectures, Müller concentrates on customs and laws (XIX), stating that ‘Nowhere can we study this growth of custom and its gradual assumption of a sacred character better than in India’ (p. 535); and sacred books (XX). Deemed as ‘sacred’ are all those books ‘formally recognized by religious communities as constituting the highest authority in matters of religion, which had received a kind of canonical sanction, and might therefore be appealed to for deciding any disputed points of faith, morality, or ceremonial’ (p. 539). He asserts that no sacred books originate in Europe; rather, all sacred books were conceived and composed in the East, namely, in five ‘birthplaces’: India, Persia, China, Palestine and Arabia. In his concluding remarks, Müller recounts the purposes of his course of lectures: to survey the materials available for the study of the origin, growth and in some cases the decay of religious ideas.