And it is precisely the prominent authorities of natural science who show by their disagreements on such matters that they are mere tyros in philosophy. It follows, then, that the socalled ethics of science, the touchstones of which the boast is made that they never fail in determining what is knowledge and what is mere conjecture, are based on a purely instinctive practice, not on a conscious theory of understanding. Although our time excels in diligent scientific research, yet the numerous differences among scientists show that they are not capable of using their knowledge with a predetermined certainty of success. Otherwise, how could misunderstandings arise? Whoever understands understanding, cannot misunderstand. It is only the absolute accuracy of astronomical computations which entitles astronomy to the name of a science. A man who can figure is at least enabled to test whether his computation is right or wrong. In the same way, the general understanding of the process of thought must furnish us with the touchstone by which we can distinguish between understanding and misunderstanding, knowledge and conjecture, truth and error, by general and irrefutable rules. Erring is human, but not scientific. Science being a human matter, errors may exist eternally, but the understanding of the process of thought will enable us quite as well to prevent errors from being offered and accepted as scientific truths as an understanding of mathematics enables us to eliminate errors from our computations. It sounds paradoxical and yet it is true: Whoever knows the general rule by which error may be distinguished from truth, and knows it as well as the rule in grammar by which a noun is distinguished from a verb, will be able to distinguish in both cases with equal certainty. Scientists as well as scribes have ever embarrassed one another by the question: What is truth? This question has been an essential object of philosophy for thousands of years. This question, like philosophy itself, is finally settled by the understanding of the faculty of human thought. In other words, the question of what constitutes truth is identical with the question of the distinction between truth and error. Philosophy is the science which has been engaged in solving this riddle, and the final solution of the riddle by the clear understanding of the process of thought also solves the question of the nature of philosophy. Hence a short glance at the nature and development of philosophy may well serve as an introduction to our study. As the word philosophy is connected with various meanings, I state at the outset that I am referring only to socalled speculative philosophy. I dispense with frequent quotations and notes of the sources of my knowledge, as anything that I may say in this respect is so well established that we can afford to discard all scientific by-work.