Swift as a shadow
Or, if there were a sympathy in choice,
War, death, or sickness did lay siege to it,
Making it momentany as a sound,
Swift as a shadow, short as any dream,
Brief as the lightning in the collied night,
That, in a spleen, unfolds both heaven and earth;
And ere a man hath power to say "Behold!"
The jaws of darkness do devour it up:
So quick bright things come to confusion.
Lysander is discoursing on true love, whose course, he has determined, "never did run smooth" [see p. 24]. Even granting that a courtship arises from "sympathy" (mutual desire) rather than family pressure, all kinds of difficulties will arise. War, death, sickness—in short, the ravages of time—assail true lovers, making their passion as "momentany" (momentary) as a sound. ("Momentany" was a common form in the sixteenth and seventeenth centuries, although Shakespeare usually avoids it.)
Lysander gets caught up in this idea and generates more prolix metaphors for the brevity of love—it is as "swift" (fleeting) as a shadow, short as a dream on a midsummer night, brief as a flash of lightning. This flash "unfolds" (displays) heaven and earth in a brief burst, only to be swallowed up by the "jaws of darkness" that are the rest of human experience. ("Collied" means "as black as coal," deriving from the same root as "collier.") For Lysander, love is a "quick bright thing" brought to confusion by the inexorable workings of time and nature.
Lysander's "swift as a shadow," the most famous phrase from this speech, ultimately derives from the proverbial expression "to flee like a shadow," which dates from around the twelfth century.