Natural grace, a term for poetic study (often in contrast to the “melancholy” poetic style), refers to free and unconstrained aesthetic style and artistic appeal in poetic works. It gives expression to the imagination of the poet, the natural and free disposition of his spirit, and his pursuit of aesthetic enjoyment. When in such a state of mind, the poet is “totally absorbed in his interaction with heaven and earth,” roaming freely in boundless time and space. The concept represents a poetic style in which the poet and what he portrays in his poem merge into a natural whole.
Du Fu could not write as freely and unconstrained as Li Bai, while the latter did not possess the style of melancholy and profoundness typical of Du Fu’s poems. (Yan Yu: Canglang’s Criticism of Poetry)