Plato famously banished poets from his ideal society, arguing that they created and perpetuated falsehoods. Sidney, in his masterful defence of poetry, takes on Plato's claim, arguing that a poet "nothing affirms, and therefore never lieth." Sidney looks at other professions and explores how they present something that is not true to be true, and then compares them to the profession of a poet, arguing that deception is never something that a poet tries to achieve through his or her art. Note what he says:
The poet never maketh any circles about your imagination, to conjure you to believe for true what he writes... And therefore, though he recount things not true, yet because he telleth them not for true, he lieth not...
Poetry does not involve deception, rather it involves the willing suspension of disbelief, which is so key to its appreciation. Sidney thus defends poetry from Plato's argument that it promotes deceit by arguing that poetry is based on the imagination. Poetry never pretends to set forth a "truth" and rather relies on the audience's willing suspension of disbelief for its success. Because the poet is open about the imagination and creativity in his poetry, he is not lying and is not trying to promote deceit.