In Per Amica Silentia Lunæ, William Butler Yeats writes:
I had no natural gift for this clear quiet, as I soon discovered, for my mind is abnormally restless; and I was seldom delighted by that sudden luminous definition of form which makes one understand almost in spite of oneself that one is not merely imagining. I therefore invented a new process. I had found that after evocation my sleep became at moments full of light and form, all that I had failed to find while awake; and I elaborated a symbolism of natural objects that I might give myself dreams during sleep, or rather visions, for they had none of the confusions of dreams, by laying upon my pillow or beside my bed certain flowers or leaves. Even to-day, after twenty years, the exaltations and messages that came to me from bits of hawthorn or some other plant seem of all moments of my life the happiest and wisest.