 In April 1661 he enrolled in his father's former university at age 14, and completed his bachelor's degree in Philosophy in December 1662.He defended his "Disputatio Metaphysica de Principio Individui" ("Metaphysical Disputation on the Principle of Individuation"), which addressed the principle of individuation, on 9 June 1663.Leibniz earned his master's degree in Philosophy on 7 February 1664.