It is one of the most puzzling aspects of Delius's creative path that, between 1914 and 1923, while also occupying himself more amenably with the composition of that most romantic of idioms, the symphonic poem, he became consumed with the need to write a series of instrumental works entirely dependent on the classical imperatives of sonata and concerto. Beecham was baffled; he considered the enterprise a failure, save for the Violin Concerto, and expressed a relief in the restoration of the composer's natural gifts in the freer forms of Eventyr, A Song Before Sunrise and, even more so, in Hassan. Heseltine was similarly hesitant: ‘Delius is not seen at his best in those works whose form is dependent upon the development of contrasted themes in a certain relation preordained by tradition’. While Evans, perhaps the most eloquent of all, remarked: ‘Where we may find them [Delius's compositional methods] to fail is in those works which by their medium invite comparison with the classics on which our musical experience is based. Even by their titles alone, the Delius concertos and sonatas compel us to apply terms of evaluation foreign to the comp us to apply terms of evaluation foreign to the composer’s nature’.

Delius never gave his reasons for wishing to revisit more classical idioms that he had only briefly explored in the past. The Violin Sonata in B major of 1892, a perhaps overblown work, had been long since shelved, as had his Suite for Violin and Orchestra; and, for all its popularity and number of performances, his Piano Concerto never met with his greatest satisfaction. Nevertheless, Delius evidently believed that the possibilities of rhapsodic lyricism and thematic variation, seminal to his style and modus operandi, might be a productive source of new forms, and, given his zeal to explore new structural ideas in his symphonic poems, the traditional sonata and concerto seemed a ripe field for his attention and one to which he evidently felt he could make a novel and creative contribution. In consequence, this gave rise to a new and heightened awareness of the role of thematic material and the integral role of key in his style. With regard to the latter, in particular, we find that Delius seemed to discover a new simplicity in his favoured one-movement forms, expressed by a new contrapuntal fluency, the choice of uncomplicated tonalities (e.g. C major, A minor, D minor and F) and the process of ‘dual functionality’ in his structural thinking.oser's nature’.