The imitation of the classics having thus become essential to literary creation, what was to be its relation to the imitation of nature? The ancient poets seemed to insist that every writer is at bottom an imitator of nature, and that he who does not imitate nature diverges from the purpose and principle of art. A lesson coming from a source so authoritative as this could not be left unheeded by the writers of the Renaissance, and the evolution of classicism may be distinguished by the changing point of view of the critics in regard to the relations between nature and art. This evolution may be traced in the neo-classical period through three distinct stages, and these three stages may be indicated by the doctrines respectively of Vida, Scaliger, and Boileau. Vida says that it is the first essential of literary art to imitate the classics. This, however, does not prevent him from warning the poet that it is his first duty to observe and copy nature:-- "Praeterea haud lateat te, nil conarier artem, Naturam nisi ut assimulet, propiusque sequatur." For Vida, however, as for the later classicists, nature is synonymous with civilized men, perhaps even further restricted to the men of the city and the court; and the study of nature was hardly more for him than close observation of the differences of human character, more especially of the external differences which result from diversity of age, rank, sex, race, profession, and which may be designated by the term _decorum_.[260] The imitation of nature even in this restricted sense Vida requires on the authority of the ancients. The modern poet should imitate nature because the great classical poets have always acknowledged her sway:-- "Hanc unam vates sibi proposuere magistram." Nature has no particular interest for Vida in itself. He accepts the classics as we accept the Scriptures; and nature is to be imitated and followed because the ancients seem to require it.