by William Shakespeare
What is your substance*, whereof are you made,
That millions of strange shadows on you tend?
Since every one hath, every one, one shade,
And you, but one, can every shadow lend.
Describe Adonis, and the counterfeit
Is poorly imitated after you;
On Helen's cheek all art of beauty set,
And you in Grecian tires** are painted new.
Speak of the spring and foison*** of the year:
The one doth shadow of your beauty show,
The other as your bounty doth appear;
And you in every blessèd shape we know.
In all external grace you have some part,
But you like none, none you, for constant heart.
**tires: a sort of Greek headdress
***foison: rich or bountiful harvest (a 14th-century word already languishing in Shakespeare's time, btw)
The short take on form and interpretation
It's a Shakespearean sonnet, of course, in iambic pentameter with the following rhyme scheme: ABABCDCDEFEFGG. The "turn" or volta occurs in the ninth line, when Shakespeare stops praising the beloved's beauty through comparisons with Adonis and Helen of Troy and starts praising the beloved's beauty through comparison to the beauty of spring and the abundance of the harvest. Throughout the poem, Shakespeare intimates that beauty found in others and in nature is a reflection of the beloved. The final couplet "turns" further still in asserting that while the beloved may be compared to other externally beautiful things, nobody matches the beloved for "constant heart."
I have to say in all honesty that, while this poem is usually considered one of the love poems in the Fair Youth sequence, it could as easily be read as praise of God. I mean, you've got a beloved whose beauty is unrivalled and can be widely shared in all things. The beauty of spring is a mere echo of the beloved's beauty, and the harvest appears as the beloved's bounty - "And you in every blessèd shape we know." I'm not saying that's how it was intended, but I am saying that I think it could be read in such a way. I'm willing to be proved wrong, if you've got an alternate position.
The longer take on interpretation
1. The first four lines.
I think this poem is lovely when read as-is, but I think the actual understanding of the poem is greatly aided by understanding the Elizabethan interest in Neoplatonism*, whence much of Shakespeare's language and imagery for this poem is drawn. The word "substance" refers to the essence or spirit of the person in question. The reference to shadows is a reference to a reflection or image, and not to the beloved's actual reflection.
*Neoplatonism is a term ascribed to groups who claim to follow the teachings of Plato, but who often interpret his teachings in a different way than he himself might have done - usually, by focusing as much on the teachings of Petrarch and/or a guy named Plotonius as on the words of Plato.
That said, Oscar Wilde evidently thought that this poem might have been addressed to an actor, and that the reference to "shadows" was to the roles played by the actor on the stage, a theory which one might bolster through reference to Shakespeare's Macbeth, Act V, sc. 5, in which he writes "Life's but a walking shadow, a poor player/That struts and frets his hour upon the stage." Still others believe that Shakespeare's reference to shadows and shades is indicative that the beloved "fair youth" was away, leaving the Bard with only a portrait and/or other reminders.
The reference to a shade, in any case, is a reference to an actual shadow cast by light, yet it has a potential double meaning of a spirit or ghost, as well as a possibility of meaning something that casts all the things around it into shadow. The playfulness of this particular term, which has a possible triple or quadruple interpretation within this particular poem, is tremendous, although not at all easy to catch unless you're willing (as I'm doing here) to break the poem down bit by bit.
The contrast between shadow and substance here is also a playful sort of notion. He's contrasting the tangible with the intangible, the original with copies, the real with the unreal. It's a very broad concept, and masterfully managed through the words chosen, although since some of the meanings are obscured, it's not as evident on the surface just how very many complimentary and contradictory concepts he's playing with simultaneously.
The second quatrain.
The next four lines discuss Adonis and Helen, and claim that the beloved is at least as lovely as the most famous of beautiful males and females, and that anyone trying to paint a portrait of either Adonis or Helen would end up with a portrait of the beloved - albeit in the case of Adonis, a poor approximation of the beloved, and in the case of Helen, the beloved would be wearing Greek headgear.
The line "On Helen's cheek all the art of beauty set" has a double meaning: On the one hand, he's saying "paint Helen" and on the other hand, he's intimating that Helen has been a "painted woman", one who wears cosmetics and enhances her beauty through artifice, whereas the beloved is so beautiful that he is painted new - probably without having to perform the Elizabethan equivalent of airbrushing or photoshopping.
The third quatrain.
The third quatrain, which extends the Platonic conceit to nature. (The term "conceit" here means "an extended metaphor", and has nothing to do with being conceited.) In this part of the poem, Shakespeare says that if the beloved is the ideal of what beauty ought to be, then anywhere beauty can be found springs from that ideal. He therefore compares the beloved to spring and harvest, finding the beloved more beautiful than what nature produces, and seeing reminders of the beloved in all things during the harvest.
The closing couplet.
In the final couplet, Shakespeare goes further still, saying that anything beautiful in the world echoes the beauty of the beloved, but that the beloved is singular in the constancy of his heart.
Or does he? I've seen it argued that perhaps Shakespeare is instead saying that the beloved likes nobody else at all, and is not a "constant heart". This is based on the idea that sonnets placed earlier in the collection (40-42) express betrayal. However, looking at the sonnets that lead up to and closely succeed this one, I have to say that I believe Shakespeare still had on his flatterer's hat when he wrote this poem and that he intended for it to be read as serious praise (although it would not be unlike him to have a purposeful double meaning that cut the other way, believing it to be his own private joke - the man engaged in double entendres and puns as readily as most of us breathe, and I can easily credit him with sneaking a double meaning in here.
Another school of thought holds that it was common for poets to encourage their "betters" to behave properly by writing of them as if they were already doing so. Under that construction, it would mean that Shakespeare was crediting the beloved with constancy and faithfulness in hopes that it would bolster the beloved's resolve to be constant and faithful.
I figure that no matter how you slice it, this is one beautiful and interesting poem.