The arguments for the authenticity of Speusippus' letter are re-evaluated. In what is a radical reassessment, it is argued that the letter is best interpreted in the context of events that took place during the reign of Perdiccas III. The 31st Socratic Letter (likely to be a genuine work of Plato) provides evidence of Plato's involvement in Macedonian affairs in the late 360's, which resulted in the hostility exhibited by Philip towards Plato and his successors. The rhetorical structure of the letter is analysed in depth thereby elucidating Speusippus' aim in addressing Philip. There follows a text and translation together with a full commentary on the letter.