[English Literature in the Sixteenth Century, Excluding Drama (OUP, 1954)] makes for very hard reading, as Lewis no doubt knew. The first few pages refer casually to Pico della Mirandola (1463–94), Marsilio Ficino (1433–99), Paracelsus [Aureolus Bombastus von Hohenheim] (1493–1541), [Heinrich Cornelius] Agrippa [von Nettesheim] (1486–1535), names barely known (if at all) to most students of English literature. A little later Lewis switches casually from the De Rerum Natura of [Bernardinus] Telesius (1509–88) to the De Rerum Sensu et Magia of [Tommaso] Campanella (1568–1639), giving no introduction to either name. Six pages later he mentions that “pleasing little tract De Nymphis”; from what Lewis says I would be interested to read it, but he gives no reference. Earlier today, a friend of mine sent me an email to inquire what I knew (if anything) about another of these unidentified quotations. This one comes from Lewis’s Experiment in Criticism. In the third chapter, Lewis writes without preamble, translation, or citation: “Zum Eckel find’ ich immer nur mich” . My friend wanted to know what this meant and whether Lewis was quoting.
The meaning is straightforward enough. I told her to translate it, “ad nauseam, I find only myself.” Lewis uses this passage almost to translate his own phrasing in the sentences coming just before: “The real objection to that way of enjoying pictures is that you never get beyond yourself. The picture, so used, can call out of you only what is already there.”
But is Lewis quoting? If he isn’t, why German? It’s reasonable to suppose he is, so I poked around a bit, and it looks like he is indeed quoting — or to be more accurate, paraphrasing. There are two clues in proximity to the passage that point the way: (1) “Arthur Rackham’s [illustrations] to The Ring […] at a time when Norse mythology was the chief interest of my life”, and immediately following the German passage, and signalling a change in subject, (2) “In music […]”. 
I think the source is the libretto to Richard Wagner’s opera, Die Walküre. In Act II, Wotan (equivalent to the Norse Odin) sings: “Zum Ekel find’ ich / ewig nur mich / in Allem, was ich erwirke!” “Only I find / Myself in all I am planning!”  As you can see, Lewis turns immediately from pictures to music in the essay, right at the moment of this paraphrase. Prior to it, he discusses Arthur Rackham’s illustration’s to Wagner’s Ring operas. These include wonderful illustrations for The Valkyrie, published in 1910, when Lewis would have been twelve years old. Lewis even mentions Valkyries directly a few pages before trotting out this German passage. It all seems to fit. The German phrase is the fulcrum in the subject matter of the chapter, making it all the more intriguing that Lewis chose to signal the shift in untranslated German. Of course, in Lewis’s day, the majority of his readers could be relied on to understand simple phrases in the most common European languages. Whether they would have gotten the reference, I’m not sure. It seems likely enough. But today, not so much.
So, mystery solved? Does anyone have an alternative theory? I do think that some of Lewis’s works could really benefit from annotated editions, along the lines of Douglas Anderson’s Annotated Hobbit. I’ve thought this before, but I’ve never undertaken any such project myself, both because I have my hands full with Tolkien, and because I know so many other scholars better qualified than I am to take on Lewis at his most obscure.
 This is from an essay called “New Learning and New Ignorance: Magia, Goeteia, and The Inklings”, given as the keynote address at the C.S. Lewis and Inklings Conference in 2006. It was later published in the collection Myth and Magic: Art according to the Inklings (ed. Seguro and Honegger, Walking Tree, 2007), but since I don’t have the collection in front of me, the quotation I give above is from the keynote paper, which Tom kindly sent me in 2006. The published quotation might be slightly different.
 C.S. Lewis. An Experiment in Criticism. Cambridge: Cambridge University Press, 1961, p. 22.
 Ibid., pp. 14–5, 22.
 Richard Wagner. Die Walküre. Trans. Charles Henry Meltzer. New York: Fred Rullman, Inc. 1904, p. 28, 29.