This article first appeared in the show programme for the our 2006 Romeo and Juliet.

The earth, that's nature's mother, is her tomb.
What is her burying, grave that is her womb...
Within the infant rind of this small flower
Poison hath residence and medicine power. 
(Friar Lawrence, Act 2 Scene 3)

Day and night, the earth as both womb and tomb, herbs and flowers that are simultaneously poisonous and medicinal, virtue and vice, God's grace and our own desires:
'such opposèd kings encamp them still
In man as well as herbs.'

Give Shakespeare an idea and he is equally interested in its opposite. Opposition is indeed the key to Romeo and Juliet: the lovers are doomed because they are from the two opposed houses of Capulet and Montague. In a violent world, violent delights have violent ends. Youthful passions boil over not only into poetry and embraces, but also into insult and sword-fight.

Friar Laurence's soliloquy cuts to the quick of the play's double vision. It is structured around the rhetorical figure of oxymoron, the paradox whereby opposites are held together. Versions of the figure recur throughout the play, from Romeo's 'heavy lightness, serious vanity' to the duet of nightingale and lark in the great scene of lovers parting at dawn.

At the beginning of the play, Romeo is in love with Rosaline. Or rather, he is in love with the idea of being in love. We never actually see Rosaline: she exists solely as the idealised love-object of Romeo. She is nothing more than a literary type, the beautiful but unavailable mistress of the sonnet tradition that goes back to the Italian Renaissance poet Petrarch. The Petrarchan lover thrives on artifice and paradox. The fire in his heart is dependent on his lady's icy maidenhood
'Feather of lead, bright smoke, cold fire, sick health!
Still-waking sleep that is not what it is!'

As the Friar recognies, this is mere 'doting', not true loving. And so long as Mercutio is around, the bubble of poetic language keeps on being pricked - is it not just a matter of rhyming 'love' with 'dove'? Romeo still poeticises on seeing Juliet, though he speaks in more richly textured imagery: 
'It seems she hangs upon the cheek of night
As a rich jewel in an Ethiop's ear.'

When the lovers meet at the Capulet ball, they weave a verbal dance that answers to the motions of their bodies and hands: their initial dialogue is wrapped into the form of a sonnet. But over the next few scenes their language evolves into something more fluid and more natural. You can hear Shakespeare growing as a poet even as you see the love between Juliet and Romeo growing from infatuation at first sight to the conviction that each has found the other's soul-mate.

Love is a chemistry that begins from a physiological transformation - Romeo is 'bewitched by the charm of looks' - but it becomes a discovery of the very core of human being: 
'Can I go forward when my heart is here?
Turn back, dull earth, and find thy centre out.'

What haunts the lover is the suspicion that it might all be a dream. Mercutio spins a tale of how love is but the mischief of Queen Mab, midwife of illusion. Romeo blesses the night, but then acknowledges his fear that:
'Being in night, all this is but a dream,
Too flattering-sweet to be substantial.'

Juliet has to deal with another fear. For a girl in Shakespeare's time, chastity was a priceless commodity. To lose her virtue without the prospect of marriage would be to lose herself. In the speech that begins 'Thou knowest the mask of night is on my face', Juliet reveals quite remarkable self-understanding. She is acutely aware that in love the stakes for a woman are far higher than those for a man. Here Shakespeare's poetic language becomes the vehicle of both argument and emotion. The artifice of rhyme is replaced by blank verse that moves with the suppleness of thought itself.

In the original production, the lines would have been spoken by a young male actor of perhaps around the same 13 years as the character of Juliet. By highlighting extreme youthfulness (in the source, Juliet is 16), Shakespeare makes a bold implicit claim for his poetic drama. Both actor and character are speaking with maturity far beyond their years: such, the dramatist implies, is the metamorphic potency of the mingled fire and powder of love and art.

Though younger than Romeo, Juliet is more knowing. She senses the danger in his talk of idolatry. In the soaring love-duet that is their final scene together before Romeo's exile, she wills the song to be that of the nightingale rather than the lark because she knows that the break of day will mean the end of their night of love and the dawn of a harsh reality in which she will be reduced to the status of a bargaining chip in the negotiations between Verona's powerful families.

According to the social code of the time, it is the duty of the young to obey the old. Marriage is a matter not of love, but of the consolidation and perpetuation of wealth and status. Arthur Brooke, author of the Tragicall Historye of Romeus and Juliet which Shakespeare had before him as he wrote, told his readers that the moral of the story was that young lovers who submit to erotic desire, neglecting the authority and advice of parents and listening instead to drunken gossips and superstitious friars, will come to a deservedly sticky end.

Shakespeare's play, by contrast, glories in the energy of youth. It does not seek to advance a moral, but offers instead the tragic paradox that the heat in the blood that animates the star-crossed lovers is the same ardour that leads young men to scrap in the street and to kill out of loyalty to their friends.

The kinship of love and revenge, the perpetual war between the generations: Shakespeare will return to this territory in later plays such as Hamlet and King Lear. The final scene takes place in an ancestral tomb, but those who lie dead are the flower of a city's youth - Mercutio, Tybalt, Paris, Juliet and her Romeo.