Handel's L'Allegro, il Penseroso ed il Moderato (1740) is a particularly striking exploration of the relationship between the human psyche and its surrounding environment. Significantly, it is one of the composer's most important and sustained settings of English poetry, using large sections of Milton's short poems L'Allegro (the ‘happy’ or ‘sanguine’ man) and Il Penseroso (the ‘melancholy’ or ‘pensive’ man). It is, however, an unusual work that is difficult to categorise with conventional terminology; for instance, it is neither an ode nor an oratorio. In Handel's L'Allegro, sections from Milton's companion poems are interwoven to form the first and second parts of an oratorio-like structure that is completed by a concluding Part III, ‘Il Moderato’ by Charles Jennens. There is neither drama nor narrative, and there are no typical dramatic characters. Instead, the libretto creates a self-examining dialogue between two human temperaments, both innate in the human psyche, of which the moderating third part was intended as the resolution. Having originally been poetical constructs, the allegorical characters L'Allegro and Il Penseroso become not only vocal beings, in the mouths of Handel's singers, but also, arguably, more separate and more vivid abstractions. The poetic landscape that surrounds them becomes an outward projection of their inner mental states as they move through a range of imaginary scenes. Unpicking how Handel's music presents L'Allegro and Il Penseroso as spectators and embodies their surrounding world – especially in the context of eighteenth-century notions of spectatorship – is a significant aid when seeking a wider understanding of the work.

Musical representation is a necessary and vital component of Handel's L'Allegro, and the fecundity of his visual imagination is intrinsic to the musical energies of the work. The quality of the music suggests that the libretto readily appealed to the composer's dramatic, visual aesthetic; from the late 1730s onwards he seems increasingly to have elected to ‘represent’ within his musical scores what had been visual in the theatrical experience of his operas. However, the task of capturing in music both the depth of allusion and the range of sensory experience in Milton's poetry was not inconsiderable. With regard to these individual characteristics, and in response to the original poems, Samuel Johnson comments: