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.