Book excerpt: The quest for unity and multiplicity is one of the most important concerns in the history of human thought. Since the origins of the history of philosophy up to the present, we can observe more or less unceasing interest in the issue. The same holds of the writings of Thomas Aquinas, to whose conception this work is devoted. Since the problem of unity and multitude is closely linked to many other key metaphysical issues, such as the doctrine of transcendental concepts, the mode of composition of being qua being, as well as substantial and accidental being, or the doctrine of whole and part, we believe that its proper interpretation not only can clarify some partial metaphysical problem, but will also contribute to understanding the metaphysical thought of the Angelic Doctor as a whole.