Sir John Suckling was the son of the secretary of state and the nephew of the lord treasurer of England. After an education on the Continent, he gained a reputation as a spendthrift, lothario, and gambler but also as an accomplished poet and playwright. Suckling may have sat for Van Dyck in 1638, the year his tragedy Aglaura was staged. By displaying Shakespeare’s First Folio in his portrait, Suckling took a stance in contemporary debates about the merits of Shakespeare and modern (as opposed to classical) poetry. A similar position is expressed with the line from the Roman satirist Persius inscribed on the boulder to the right of Suckling: NE TE QUÆSIVERIS EXTRA (Do not seek outside yourself).
Source: Paintings in The Frick Collection: American, British, Dutch, Flemish and German. Volume I. New York: The Frick Collection, 1968.