Almost eight hundred years have passed since St. Thomas Aquinas made his marvelous synthesis. The encounter of Western thought with Aristotelianism allowed him to elaborate a metaphysics that still has answers. He was not a mere repetition of Aristotle. On the contrary, the Angelic Doctor understood, explained and enriched him. Moreover, he provided Christian revelation with a solid armor against fideism. Since then, Thomism has provided us with solid intellectual resources to think and reflect correctly on every aspect of reality. Contemporary man needs this particular vision. To shake off the inherited prejudices that consider medieval thought as purely theological, obscurantist, and other similar slogans. A contempt skillfully articulated by a materialistic philosophy, devoid of any transcendence and incapable of taking charge of this drifting Western civilization. I originally wrote this book in Spanish and translated it into several languages. It is the first of a series that will deepen each of the topics studied here. The Series has thirteen volumes and is complete in Spanish, English, Portuguese, French, and Italian. My goal is to offer a brief, clear, and concrete vision of Aquinas' philosophical thought. The structure of the book is as follows: "Some Preliminary Words. Prologue. Introduction. 1-The First Principles. 2-The Principle of Contradiction. 3-The Being. 4-The Entity. 5-The Analogy. 6-The Transcendentals. 7-The Categories. 8-The Substance. 9-The Accidents. 10-Act and Potency. 11-Essence and Act of Being. 12-The Causality. 13-God. 14-The Human Soul. 15-The Angels. 16-The Twenty-Four Thomistic Theses. By way of an Epilogue." Finally, I would like to point out that it contains many quotations from recognized authors: Garrigou-Lagrange, Gardeil, Manser Gallus, Grenier, Hugon, etc.