A furtive cousin of the untraceable allusion is the unconscious allusion. A poet writes a line that turns out to be an echo, or even a facsimile, of something he or she has long ago read or heard and then has apparently stored away and forgotten, like a squirrel with too many caches of nuts. A line in a poem I wrote in 1978 about my wedding called the grass “green as glass”, a phrase I was abashed to find I had apparently borrowed from a Golden Book, The Color Kittens, which my mother read to me before I could read to myself – that is, before about 1954. In another poem, written later, the image “the bath of silence”, which at first I thought I’d come up with all by myself, turned out to owe something, even if not its precise phrasing, to the description in George MacDonald’s The Princess and the Goblin, one of the first books I could and repeatedly did read to myself. The Princess’s great-great-grandmother lays the little girl down in an apparently bottomless blue bath, an otherworldly image both alluring and alarming. Recently, though, I came upon a claim that the phrase “bath of silence” originates with Meister Eckhart; both unconscious and untraceable, perhaps.
-- Rachel Hadas, TLS, February 1, 2013