Keats also reflected on the background and composition of his poetry, and specific letters often coincide with or anticipate the poems they describe. In February to May 1819 he produced many of his finest letters. Writing to his brother George, Keats explored the idea of the world as "the vale of Soul-making", anticipating the great odes that he would write some months later. In the letters, Keats coined ideas such as the Mansion of Many Apartments and the Chameleon Poet, concepts that came to gain common currency and capture the public imagination, despite only making single appearances as phrases in his correspondence. The poetical mind, Keats argued: