Henri Focillon, from The Life of Forms (1948):

What constitutes style? Firts, its formal elements [...] which make up its repertory, its vocabulary [...] Second, altough less obviously, its system of relationships, its syntax.

Gottfried Semper, from Der Stil (1860):

Art has its particular language, residing in formal types and symbols which transform themselves in the most diverse ways with the movement of culture through history, so that in the way in which it makes itself intelligible, an immense variety prevails, as in the case of language. Just as in the most recent research into linguistics, tha aim has been to uncover the common components of different human linguistic forms to follow the transformation of words [...] taking them back to one or more starting points [...] a similar enterprise is justified in the case of the field of artistic inquiry.