In the Meditations Olivier Messiaen uses a complicated semiotic system that is made up of elements including the langage communicable, birdsong, plainsong, specific musical techniques such as the modes of limited transposition, Greek and Hindu rhythms, superscriptions and prefatory material. Messiaen selected texts from the Summa Theologiae, but the syntax of his language was not sophisticated enough for him to attempt a literal translation; he therefore had to resort to paraphrase. In addition to the Rosetta Stone, another clue that helps to explain the language system within the Meditations is Messiaen's discussion of communication by angels. The reference to angels in the preface is instead based on St Thomas Aquinas writings on 'The language of the angels' and on Messiaen's highly developed personal theology. Any theological explanation of Messiaen's music must begin with the composer's own comments, which are so fluent and detailed that they are, as Weir describes them, 'a stimulus actually to enter his perfumed and rainbow-coloured world'.