The Seventh Letter is a literary and philosophical text of the mid-fourth century BC (ca. 360 BC), the authorship of which has long been disputed by classical scholars. Much contemporary scholarly opinion favors the authenticity of the text—i.e. that it was written by Plato himself, or by a student of Plato who possessed direct and intimate knowledge of the events and circumstances involved.

For readers and students of Plato and Greek philosophy, interest in the Seventh Letter lies in the text's account of Plato's involvement with Dionysius II (Dionysius the Younger), tyrant of Syracuse. The philosopher Dion, the tyrant's uncle, convinced his nephew to invite Plato to Syracuse to serve as a tutor to the young Dionysius in philosophy and political ethics (much as Aristotle would later serve as tutor to Alexander the Great). Plato made three journeys to Syracuse, but became victimized by court intrigues, especially involving the exile of Dion. Dionysius was at times so enthusiastic about Plato that he kept the philosopher a virtual prisoner; at other times Plato feared he might be murdered by the tyrant's fractious court. Plato even had difficulties escaping from Syracuse back to Athens.

Plato's involvement with Dionysius II of Syracuse has attracted attention as the philosopher's attempt, apparently his sole attempt, to apply his idealistic political philosophy to real-world politics; and its general failure has struck some critics as a negative commentary on the practical applicability of a Platonic system.

The Seventh Letter also has a detailed exposition of Plato's doctrine of the Forms. Toward the end of the letter is an explanation about the perfect circle as an existing, unchanging, and eternal Form, and how any reproduction of a circle is not the perfect circle. The Form of a perfect circle cannot even be talked about, because language and definitions are not the perfect circle either.

Plato begins to talk about the Forms in response to the false claims that Dionysios has about the full spirit of philosophy and the full understanding of Plato's philosophical discoveries. Plato argues against Dionysios by giving his best account of what he thinks about the true and real circle of geometry, unlike the words, drawings, definitions, and knowledge about what truly exists. The actual, intelligible circle cannot be expressed in changing language. Plato is attempting to describe what is eternal and unchanging, not becoming. Plato is contrasting being with becoming. However, Plato claims that even he cannot adequately write about the Forms in language, and that it is best to not write about the topic. It is the dialectic method through oral questions and answers that is best for one to arrive at a divine spark of understanding about the perfect Forms as they truly exist.