In his first years as a lecturer at the University of Paris (1252-1256), Aquinas wrote two short treatises at the request of his fellow friars, On the Principles of Nature and On Being and Essence. Each became widely popular, and with good reason, since each offers a succinct and illuminating summary of Aquinas's core philosophical doctrines. The first, as we have seen in Chapter 2, describes his explanatory framework of form, matter, and the four causes. Likewise fundamental, On Being and Essence outlines Aquinas's metaphysical framework. The topic of this treatise is the nature of being, in God and creatures, a subject that turns out to embrace a wide range of central metaphysical questions, such as the nature of substance, the individuation of substances, the relationship between essential and accidental properties, and the problem of universals.