From his website, Interesting Literature:
” ‘My heart leaps up’, sometimes known as ‘The Rainbow’ is perhaps William Wordsworth’s shortest great poem. In just nine lines, Wordsworth expresses a number of the several features of Romanticism: a love of nature, the relationship between the natural world and the individual self, and the importance of childhood in making the poet the man he becomes, memorably expressed by Wordsworth’s statement that ‘The child is father of the man’.
My heart leaps up when I behold
A rainbow in the sky:
So was it when my life began;
So is it now I am a man;
So be it when I shall grow old,
Or let me die!
The Child is father of the Man;
And I could wish my days to be
Bound each to each by natural piety.
Wordsworth observes a rainbow in the sky and is filled with joy at the sight of a rainbow: a joy that was there when Wordsworth was very young, is still there now he has attained adulthood, and – he trusts – will be with him until the end of his days. If he loses this thrilling sense of wonder, what would be the point of living? In summary, this is the essence of ‘My heart leaps up’.
The paradox of the line ‘The Child is father of the Man’ is that our childhoods shape our adulthoods: the inversion of the usual idea of things (that an adult man is a father to his child) neatly embodies Romanticism’s desire to shake up the way we view ourselves, and to (an idea expressed before Romanticism, notably in Henry Vaughan’s fine poem ‘The Retreat’; but it was Wordsworth and the Romantics who made the idea a central part of their worldview). Later in the nineteenth century (Wordsworth’s poem was published in 1807), the poet Gerard Manley Hopkins would take exception to Wordsworth’s insistence that ‘the child is father of the man’.
This is the beginning of the nineteenth-century worship of the child (a form of veneration arguably still with us), which will lead to Victorian literature’s Golden Age of children’s literature and also a shift in the way the concept of ‘childhood’ and ‘the child’ is viewed by society (leading to reforms in child-labour, for instance, some of these changes influenced by literature, such as Kingsley’s The Water-Babies).
‘My heart leaps up’ is a small slice of Romanticism which says more about that movement than many longer poems do. At the other end, we have Wordsworth’s vast autobiographical poem, The Prelude (for which Wordsworth recycled his above lines about the child being father of the man). But another ‘prelude’, almost a manifesto, for Romanticism can be found here in this short rainbow poem.”