Metaphysics is generally considered a ‘universal science’, insofar as it is thought to deal with all that exists in terms of being. Since Aristotle there has been a debate over how the universality of metaphysics can be secured. But with regard to it the focus was mainly put on the question whether ? and, if affirmative - how God is included in the object of metaphysics. This way of discussing the matter, however, very often neglects to consider so called ‘ens rationis’, which is said to exist in the mind. Faced with this situation, the objective of this article is to take glace at the history of scholastic philosophy so as to see whether in the scholastics philosophy, the ens rationis, a kind of being which is taken for non-being in the sense of real being, but is nevertheless not non-being in that it exists in a certain way, that is, exists in the mind, is dealt with as an object of metaphysics, and whether the metaphysics is, therefore, actually considered a science which deals with all beings insofar as they are being. With regard to it, this article investigates the theories of medieval philosophers together with some modern scholars, focusing on the question whether the concept of being including ens rationis was developed in the scholastic philosophy, and is treated as the object of metaphysics. It thereby is expected to have a chance to think about the way the universal science which considers all things can become possible, to put it another way, about the possibility of the universal science.