This work represents an investigation of the most important properties of the human mind consciousness, apperception and reflection - and of their significance for Leibnizian philosophy. The development of Leibniz's thinking in the course of his treatment of these themes receives especially detailed treatment, and is thoroughly documented on the basis of the original texts. The concepts of consciousness and reflection were the object of intensive discussion in the l7th century. Starting out from the problem of the distinction between humans and brutes - Descartes' view of animals as mere machines was always decisively rejected by Leibniz-Kulstad shows the significance of these concepts in the early writings of Leibniz. He shows how Leibniz was then stimulated by Locke to add the word "apperception" into his philosophy. The author sets out the influence of Locke on Leibniz and demonstrates how Leibniz adopted a firmer and more constant position as to the relation between consciousness and reflection than one finds in Locke's own writings. From the beginning to the end of his life Leibniz defends the thesis that both consciousness and reflection consist in the memory of one mental act via another. The author shows how Leibniz hereby aligns himself with an European philosophihical tradition which can be traced back to Plato, Aristotle and Aquinas. Of course, a clarification of the meanings of words such as "consciousness", "reflection" and "apperception" is important not only for an understanding of Leibniz's philosophy but also for contemporary metaphysics and theory of knowledge. Leibniz certainly recognized and thought through the problems associated with these words, but he never developed a final, coherent theory, a fact which certainly reflects in part the complexity of the underlying problems. By exploiting not only all the relevant Leibnizian writings but also the results of more receent philosophy in this field, Kulstad is able to draw a reliable picture both of Leibniz's treatment of these problems and of the influence of his views on his contemporaries and successors.