Gottfried Wilhelm Leibniz (1646-1716) is one of the greatest thinkers of the 17th and 18th centuries, a German polymath, who represents a significant name in the history of mathematics and philosophy. As a prominent mathematician, he is credited for the development of the differential and integral calculus, to which he came in the year 1675 independently from Sir Isaac Newton. In the domain of mathematics he is also credited for the discovery of the law of continuity and transcendent law of homogeneity, as well as for the invention of the so-called Leibniz wheel that greatly improved the functioning and use of the calculator as well as being responsible for the refinement of the binary number system. Many of his ideas have found their application only in the 20th century in the field of linguistics, probability theory, and computer science along with outstanding contributions to physics, geology, law, biology, medicine as well as in numerous other fields. As a philosopher, he is the most significant representative of the Rationalist stream of thinking which represents, along with the rival Empiricist, the most important schools of thought in the modern age pre-Kantian philosophy. As a scientist, and first of all a mathematician, his philosophical reflection was always expressed in extremely analytical and distinct terminology, having on his mind the scientific, especially physicist, achievements of his era. On the other hand, as a devout Christian, who was familiar with the Scholastic and ancient thinkers, he looked at reality theologically and conceptually. As a mathematician and physicist, and as a worshipper of the premodern philosophical heritage, he aligned himself with the camp of rationalist thinkers such as Descartes and Spinoza, reflecting on reality by giving precedence above all to that which is necessary instead of that which is contingent, deducing reality from the logical principles based on the truths of mind instead of truths of fact. By anticipating in his works the contemporary developments in logic and specifically analytical philosophy, he caused great veneration and admiration of later philosophers, such as Gottlob Frege, who had considered his knowledge as unrivalled by any other thinker due to the great quantity of original and innovative ideas. The aim of this work is to perform a thorough extrapolation of the largest and most creative part of his philosophy, and that is his metaphysics in which he fused in a very interesting way the two largest intellectual influences of his life, the natural sciences and ancient-medieval philosophy, and gave his response to the biggest philosophical questions and problems of his era, confronting both his Rationalist as much his Empiricist contemporaries on these issues.