Plato's unwritten doctrines

Plato's unwritten doctrines are metaphysical theories ascribed to him by his students and other ancient philosophers but not clearly formulated in his writings. In recent research, they are sometimes known as Plato's 'principle theory' (German: Prinzipienlehre) because they involve two fundamental principles from which the rest of the system derives.

The main sources ascribing esoteric doctrines to Plato are first-generation members of his Academy, most notably Aristotle. These sources indicate that Plato believed certain parts of his teachings could not be explained in writing in a way that would be accessible to general readers; therefore, he restricted their teaching to students in the Academy.

In the middle of the twentieth century, historians of philosophy initiated a wide-ranging project aiming at systematically reconstructing the foundations of the unwritten doctrines. The group of researchers who led this investigation came to be called the 'Tübingen School' (in German: Tübinger Platonschule), because some of its leading members were based at the University of Tübingen in southern Germany. While its advocates have described the Tübingen reconstruction as a 'paradigm shift' in Plato studies, critics of the Tübingen School have argued that the available evidence is insufficient to establish that Plato deliberately withheld a systematic metaphysical doctrine from his written dialogues.