Gottfried Wilhelm Leibniz (1646-1716) is familiar to us through his symbolism of calculus, as the inventor of binary numbers, which today form the basis of every computer, and as a proponent of the relativity of space and time. In contrast, his metaphysics of windowless monads that reflect the world appears just as foreign today as his theodicy, which defends the goodness of God in the face of the evils of this world. To elucidate these theses, the introduction of Leibniz's thoughts on a universal harmony and the great principles regarding his theories of concepts and truth leads to those central problems of his metaphysics. Their merit and their conditionality in his time become as visible as their significance for the present.