The allusion depends as well on the author's intent; a reader may search out parallels to a figure of speech or a passage, of which the author was unaware, and offer them as unconscious allusions-coincidences that a critic might not find illuminating.
William Irwin remarks that allusion moves in only one direction: "If A alludes to B, then B does not allude to A. The Bible does not allude to Shakespeare, though Shakespeare may allude to the Bible."