In the "Republic" the highest form is identified as the Form of the Good, the source of all other Forms, which could be known by reason. In the "Sophist", a later work, the Forms "being", "sameness" and "difference" are listed among the primordial "Great Kinds". Plato established the Academy, and in the 3rd century BC, Arcesilaus adopted academic skepticism, which became a central tenet of the school until 90 BC when Antiochus added Stoic elements, rejected skepticism, and began a period known as Middle Platonism.