This is a deeply spiritual song, even a religious one.
Even though "love" is in the title, this is not romantic love that is being spoken of, asked for "proof" of, but... love itself, as a concept.
"Does love exist?" the speaker asks of God. "Well, then... prove it!"
The first part of a song describes a journey. The speaker has no "guide," but sets out anyway. Oh, and it is late in the day-- or perhaps late in life-- so he wants to set out "before the bells of twilight peal" (those would be the "vespers," for those unfamiliar with Catholic timekeeping; the ones in the morning are "matins," as explained in the nursery rhyme "Frere Jacques.").
We're going to guess the "twilight" reference means speaker is older, as the first line is "Begin again-- no easy trick." So he has begun before, and now has to start all over.
At least the road is easy, a "spiral" downhill path (this could be a reference to "Spiral Highway," an obscure Simon song from his One-Trick Pony film that did not end up on the soundtrack).
But is the road easy? It reminds him of a coiled "serpent." Not only does he not have a "walking stick," he doesn't have any food! Just a "teaspoon of desire for [his] meal." Also, "the road is steep/ The air is thin." It seems less like he's on a hill than a mountain.
So, this is not an easy downhill path. This is more of a "it's all downhill from here" path. In fact the whole "town" is downhill, which implies that others face this same fate.
At this point, our speaker needs a shot of faith. So he prays, crying: "I trade my tears to ask The Lord for proof of love." He's not even sure he believes in God, or if there are only "stars" above him (a far cry from the prayer offered at the end of the song "Duncan"-- which also mentions "stars" and "The Lord," which Simon has begun playing again in concert; in fact, a new recording of it is included on the deluxe version of this very album).
To his own prayer, he says, "Amen." Or it may be others in the town who testify to his prayer.
Then... he seems to receive an answer to his prayer! Only, it does not come from Heaven above but from "inside [his] skin." It is a response of consolation. "Your days won't end with night," it reassures, "Let your body heal its pain."
Another clue. This "road" is the road to recovery, perhaps from an illness or injury. That explains "begin again." In physical therapy, a person may have to re-learn skills mastered as a child, like speaking or walking.
What does the voice recommend as treatment? An injection of Nature, first: "Feel the sun/ Drink the rain." Next, a large dose of faith: "bathe beneath a waterfall of light."
This time, he tearfully prays for proof of love to know "what my dreams are made of" (The original line for Shakespeare's The Tempest is "we are such stuff as dreams are made on.") He wants to know that his dreams are made of something substantial and good, not flimsy and false. If there is love, he can dare to hope.
He has kept walking downward, meantime, and now he can see "the valley below." It is, he sees, "an ocean of debris." Not the answer he was banking on.
OK, time for another prayer. No tears this time. At long last-- does love exist? "Love is all I seek!" he says, and says again. Now, he is out of tears, and out of "words." So he turns to "music" to express himself.
He is exhausted with walking and worry. Spent, he lays down by a "white oak tree." He has had it. He asked for proof of love, and got a treacherous road that led him to a valley full of rubble. He was supposed to find nourishing sunshine and rain, and only has hunger and "pain" and not even enough "air"!
He as much as dares death to come-- "No deadly nightshade, belladonna, dare lay a leaf on me." Nightshade is a poisonous plant; its Latin name is "atropa belladonna."
But the night is "silent." It is "still as prayer." And it's not "dark," either. In fact, "Darkness fills with light/ Love on Earth is everywhere." Beautiful.
But let's unpack it anyway. If he wanted to get underway before "twilight," and that implies that he was old or, as we learn later, near death, if due to not age but ailment... what does "darkness" mean? It means, well, death.
One prayer was answered by a sense of calm, the promise that his days would not end with night, and that he would bathe in a waterfall of light. If this is his death, then that promise comes true. His days don't end with "night" but with "light," and if the light fills the world, it is certainly enough to bathe in.
There is an alternate interpretation I would like to offer, though. It is foreshadowed by the reference to "bells of twilight." It's possible that this song is not just religious-- it's possible to read it so that it is about religion itself.
Some clues: A man is coming down a mountain. He doesn't have his usual "stick" with him. He is talking to God on the way. There is a reference to a "serpent." When he gets to the bottom, the valley, he finds disaster.
Might this be about... Moses? Walking down the mountainside of Sinai? And then coming to the bottom to find a pit of idolatry. (Moses turns his stick, or staff, into a serpent more than once.)
The next clue: "Silent night." Or should we say, "Silent Night." Simon already recorded that carol, so he knows it well, and it's about the birth of Jesus. A "tree" is a common metaphor for the Cross, too. So Moses asks for proof of love, and gets it, in Jesus.
I'm not saying this is what Simon means to say. But it is possible to read this interpretation into the song.
More likely, the song is about someone dying peacefully after suffering from cancer or a stroke or something, after there had been some initial hope of recovery.
We get some information as to the symptoms. Likely there was some blindness involved, requiring a "guide" or "stick" to help him find his way; now, he is supposed to be able to walk without that. He can see again, but has been spending much time in bed, gazing upward at what he hoped was more than just "stars." Also, he has trouble breathing ("the air is thin"), and at the end had trouble speaking ("words desert me").
He decides to try natural remedies ("drink the rain"), like a sunlamp ("a waterfall of light") and to "let [his] body heal" itself. He doesn't want toxic chemotherapy, or some poison that will kill him quicker ("no deadly nightshade"). But all he gets is sicker ("an ocean of debris"). Then he dies, and in death finds peace, and light, and love.
What is our the proof, then, of love? Of all things, it may be death. Dying is terrible-- painful, frightening. It takes you apart, piece by piece. But then you get to die.
And rest. In peace.
(NOTE: It would be interesting to contrast this song with Simon's similarly named "Proof," which contained the repeated lines: "Faith is an island in the setting sun/ Proof is the bottom line for everyone.")
Next Song: The Riverbank