Council of Trent: Session XXII, Sacrifice of the Mass, Chapter V

CHAPTER V. On the solemn ceremonies of the Sacrifice of the Mass.

And whereas such is the nature of man, that, without external helps, he cannot easily be raised to the meditation of divine things; therefore has holy Mother Church instituted certain rites, to wit that certain things be pronounced in the mass in a low, and others in a louder, tone. She has likewise employed ceremonies, such as mystic benedictions, lights, incense, vestments, and many other things of this kind, derived from an apostolical discipline and tradition, whereby both the majesty of so great a sacrifice might be recommended, and the minds of the faithful be excited, by those visible signs of religion and piety, to the contemplation of those most sublime things which are hidden in this sacrifice.

Translation: J. Waterworth, The Canons and Decrees of the Council of Trent (1848), Hanover Historical Texts Project transcription (1995).