The work deals with theory of common nature and universals of Aquinas in the treatise De ente et essentia. In first part is defined conceptual foundation based on Aristotelian and Avicennian concepts of essence and existence. Avicenna defines two ways how can be essence considered. First it is absolute consideration of the essence and second the essence considered as it has being. Thomas Aquinas develops his notion of essence in a quite noticeable Avicennian framework. In the second part are presented the theory of common nature by St. Thomas. The theory is based on the essence considered as it has being, we can found it in rational soul.