Note to Twelfth Night, 4.1.62, "Let fancy still my sense in Lethe steep"

A beautiful lady is in love with Sebastian and wants to take him into her house and take care of him. But he's never seen her before, so it must all be in his imagination ("fancy"). But if it is only his imagination, he hopes that his imagination will soak ("steep") his "sense" (of reality) in the river of forgetfulness ("Lethe"), so that he will forget reality and live forever ("still") in this wonderful realm of imagination. All of this is said more economically in the next line: "If it be thus to dream, still let me sleep!"

The Waters of Lethe by Thomas Benjamin Kennington