Astrophil and Stella, Sonnet 15 (You that do search for every purling spring) is written by Sir Philip Sidney.
This sonnet is addressed to contemporary protestors who seek the aid of the Muses and search for inspiration from the poetry of the classical poets and their stock images. Thus they write imitating the style and modes of the ancients and use as images all objects on Mount Olympus and from the waters flowing of from the fountain Parnassus, whether sweet or bitter.
And then there are others who use the dictionary methods to choose and select words to make noisy rhymes, and others who follow Petrarch in expressing their feelings in the conventional mode, though the great poet is long dead. Sidney seems to criticize contemporary poets like Gascoigne and Wharton (without naming them) who make extensive use of dictionaries to find suitable words for their rhymes. The result is awkward and rhymes make rattling sounds. He says that they are mere plodders and lack invention. (Sidney calls them almost dullards devoid or bankrupt of creative faculties).
Sidney goes on to mention those who still continue to sing their woes as Petrarch who is long dead; such poets lack invention and are mere imitators. Their poems are full of images of sighs, woes (taken from Petrarch), stolen from the great poet though they wish to pass for great ‘wits’. But they are wrong to seek the far-fetched help of others in a slavish manner.
But (perhaps they are unaware) the stolen (borrowed) parts get revealed quickly; such poets lack inspiration, real or true feeling. Their process of getting fame with the help of borrowed material comes to nothing. They stand exposed.
In the last lines, the poet sums up his ideas. Look within and start writing – they should look at the beautiful (breastfull) image of their Stella (beloved), real or imagined and then write — because there lies the real source of inspiration. Once again Sidney brings Stella before the readers for she is equated with the classical Muses. She is a better, rather true, source of inspiration.