The expression 'unwritten doctrines' (in Greek: ἄγραφα δόγματα, ágrapha dógmata) refers to doctrines of Plato taught inside his school and was first used by his student Aristotle. In his treatise on physics, he wrote that Plato had used a concept in one dialogue differently than 'in the so-called unwritten doctrines.'

Aristotle, Physics, 209b13–15.

Konrad Gaiser: Prinzipientheorie bei Platon

Konrad Gaiser
Platons Ungeschriebene Lehre.

Christina Schefer: Platons unsagbare Erfahrung, Basel 2001

Konrad Gaiser: Plato’s enigmatic lecture ‚On the Good‘.

German philosopher Wilhelm Gottlieb Tennemann proposed in his 1792–95 System of Plato's Philosophy that Plato had never intended that his philosophy should be entirely represented in written form.

1808, August Boeckh, who later became a well-known Greek scholar, stated in an edition of Schleiermacher's Plato translations that he did not find the arguments against the unwritten doctrines persuasive.