Comprehensible, which they call the criterion of facts, and which is produced by a real object, and is, therefore, at the same time conformable to that object; Incomprehensible, which has no relation to any real object, or else, if it has any such relation, does not correspond to it, being but a vague and indistinct representation. Ibn Sina (11th century) In the 11th century, the theory of tabula rasa was developed more clearly by Ibn Sina. He argued that the "human intellect at birth resembled a tabula rasa, a pure potentiality that is actualized through education and comes to know." Thus, according to Ibn Sina, knowledge is attained through "empirical familiarity with objects in this world from which one abstracts universal concepts," which develops through a "syllogistic method of reasoning; observations lead to propositional statements, which when compounded lead to further abstract concepts." He further argued that the intellect itself "possesses levels of development from the static/material intellect, that potentiality can acquire knowledge to the active intellect, the state of the human intellect at conjunction with the perfect source of knowledge." Ibn Tufail (12th century) In the 12th century, the Andalusian-Islamic philosopher and novelist, Ibn Tufail (known as Abubacer or Ebn Tophail in the West) demonstrated the theory of tabula rasa as a thought experiment through his Arabic philosophical novel, Hayy ibn Yaqdhan, in which he depicts the development of the mind of a feral child "from a tabula rasa to that of an adult, in complete isolation from society" on a desert island, through experience alone. The Latin translation of his philosophical novel, entitled Philosophus Autodidactus, published by Edward Pococke the Younger in 1671, had an influence on John Locke's formulation of tabula rasa in An Essay Concerning Human Understanding. Aquinas (13th century) In the 13th century, St. Thomas Aquinas brought the Aristotelian and Avicennian notions to the forefront of Christian thought. These notions sharply contrasted with the previously-held Platonic notions of the human mind as an entity that pre-existed somewhere in the heavens, before being sent down to join a body here on Earth (cf. Plato's Phaedo and Apology, as well as others). St. Bonaventure (also 13th century) was one of the fiercest intellectual opponents of Aquinas, offering some of the strongest arguments toward the Platonic idea of the mind. Descartes (17th century) Descartes, in his work The Search for Truth by Natural Light, summarizes an empiricist view