Although the first edition was published in 1855, Whitman spent most of his professional life writing and re-writing Leaves of Grass, revising it multiple times until his death. The poems are loosely connected, with each representing Whitman's celebration of his philosophy of life and humanity. This book is notable for its discussion of delight in sensual pleasures during a time when such candid displays were considered immoral. Leaves of Grass was highly controversial during its time for its explicit sexual imagery, and Whitman was subject to derision by many contemporary critics. Over time, however, the collection has infiltrated popular culture and been recognized as one of the central works of American poetry.