A Secret Vice is the title of a lecture written by J. R. R. Tolkien

J. R. R. Tolkien

John Ronald Reuel Tolkien, CBE was an English writer, poet, philologist, and university professor, best known as the author of the classic high fantasy works The Hobbit, The Lord of the Rings, and The Silmarillion.Tolkien was Rawlinson and Bosworth Professor of Anglo-Saxon at Pembroke College,...

 in 1931, given at a conference. Some twenty years later, Tolkien revised the manuscript for a second presentation.

It deals with constructed languages in general, and the relation of a mythology

Mythology

The term mythology can refer either to the study of myths, or to a body or collection of myths. As examples, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece...

 to its language. Tolkien contrasts international auxiliary language

International auxiliary language

An international auxiliary language or interlanguage is a language meant for communication between people from different nations who do not share a common native language...

s with artistic language

Artistic language

An artistic language is a constructed language designed for aesthetic pleasure. Unlike engineered languages or auxiliary languages, artistic languages usually have irregular grammar systems, much like natural languages. Many are designed within the context of fictional worlds, such as J. R. R....

s constructed for aesthetic pleasure.
Tolkien also discusses phonaesthetics, citing Greek, Finnish, and Welsh as examples of "languages which have a very characteristic and in their different ways beautiful word-form".

Tolkien's opinion of the relation of mythology

Mythology

The term mythology can refer either to the study of myths, or to a body or collection of myths. As examples, comparative mythology is the study of connections between myths from different cultures, whereas Greek mythology is the body of myths from ancient Greece...

 and language is reflected in examples cited in Qenya and Noldorin, the predecessors of Quenya

Quenya

Quenya is a fictional language devised by J. R. R. Tolkien, and used in his Secondary world, often called Middle-earth.Quenya is one of the many Elvish languages spoken by the immortal Elves, called Quendi in Quenya. The tongue actually called Quenya was in origin the speech of two clans of Elves...

 and Sindarin

Sindarin

Sindarin is a fictional language devised by J. R. R. Tolkien, and used in his secondary world, often called Middle-earth.Sindarin is one of the many languages spoken by the immortal Elves, called the Eledhrim or Edhellim in Sindarin....

. The essay contains three Qenya poems, Oilima Markirya ("The Last Ark"), Nieninque, and Earendel, as well as an eight-line passage in Noldorin.

One of the most frequently quoted passageshttp://www.google.com/search?sourceid=navclient&ie=UTF-8&rls=GGLD,GGLD:2003-48,GGLD:en&q=%22I+shall+express+the+accusative+case+by+a+prefix%22 from the essay comes in a context where Tolkien relates how he randomly met a fellow language inventor in the army:

The man next to me said suddenly in a dreamy voice: 'Yes, I think I shall express the accusative case by a prefix!' A memorable remark!

...Just consider the splendour of the words! I shall express the accusative case.' Magnificent! Not 'it is expressed', nor even the more shambling 'it is sometimes expressed', nor the grim 'you must learn how it is expressed'. What a pondering of alternatives within one's choice before the final decision in favour of the daring and unusual prefix, so personal, so attractive; the final solution of some element in a design that had hitherto proved refractory. Here were no base considerations of the 'practical', the easiest for the 'modern mind', or for the million - only a question of taste, a satisfaction of a personal pleasure, a private sense of fitness.