Plato's so-called 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. Plato is thought to have orally expounded these doctrines to Aristotle and the other students in the Academy and they were afterwards transmitted to later generations. The credibility of the sources that ascribe these doctrines to Plato is controversial. They indicate that Plato believed certain parts of his teachings were not suitable for open publication. Since these doctrines could not be explained in writing in a way that would be accessible to general readers, their dissemination would lead to misunderstandings. Plato therefore supposedly limited himself to teaching the unwritten doctrines to his more advanced students in the Academy. The surviving evidence for the content of the unwritten doctrines is thought to derive from this oral teaching. 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, which became well-known among classicists and historians, 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. On the other hand, numerous scholars had serious reservations about the project or even condemned it altogether. Many critics thought the evidence and sources used in the Tübingen reconstruction were insufficient. Others even contested the existence of the unwritten doctrines or at least doubted their systematic character and considered them mere tentative proposals. The intense and sometimes polemical disputes between the advocates and critics of the Tübingen School were conducted on both sides with great energy. Advocates suggested it amounted to a 'paradigm shift' in Plato studies. Source: Wikipedia (en)