In "Sir Gawain", the Green Knight is so called because his skin and clothes are green. The meaning of his greenness has puzzled scholars since the discovery of the poem, who identify him variously as the Green Man, a vegetation being of medieval art; a recollection of a figure from Celtic mythology; a Christian symbol; or the Devil himself. The medievalist C. S. Lewis said the character was "as vivid and concrete as any image in literature." J. R. R. Tolkien called him the "most difficult character" to interpret in the introduction to his edition of "Sir Gawain and the Green Knight". His major role in Arthurian literature includes being a judge and tester of knights, and as such the other characters consider him as friendly but terrifying and somewhat mysterious.