The point of view from which man has regarded nature for thousands of years up to modern times has been such as to promote most effectually the development of superstition; for the idea that a satisfactory insight into the character of natural phenomena can be obtained only by means of adequate experiments, and of observation perfected by the employment of the inductive reasoning and ingenious instruments, is comparatively recent. Natural science applying such means is scarcely two hundred years old. Fit instruments for the observation of nature existed only to a limited extent up to the eighteenth century, and, besides, their complete efficiency left much to be desired. The attempts to wrest from Nature her secrets by means of experiment were but feeble and unsuccessful. Altho the ancients, as is shown by the writings of Hippocrates, Galen, and others, had some knowledge of vivisection, they had practised it to a most limited extent. During the middle ages and the period of the Renaissance comparatively few physical experiments were made. Whatever researches in natural science were then undertaken were intended much less for the investigation of nature than for fantastic and superstitious purposes—as, for instance, the investigations of alchemy and astrology. It is quite obvious that, under such circumstances, a number of superficial, imperfect, and distorted observations crept into the theoretic system of natural science. However, this was not all; the diagnostico-theoretical method, by means of which antiquity, the middle ages, and even the greatest part of more modern times, had seen the natural sciences treated, was radically wrong. Man did not feel his way carefully from experiment to experiment, from observation to observation, until the general principle was found which inductively comprised a number of phenomena under one uniform principle of law, but the principle which was at the bottom of phenomena was fixed upon a speculative basis, and in accordance with this principle the phenomena were interpreted—as was done, for instance, in medicine in the case of humoral pathology. And as this speculatively constructed principle was obtained exclusively by a method dangerous to the cognition of natural sciences, by conclusion from analogy, naturally the most fantastic and adventurous conceptions soon became accepted in the realm of natural philosophy. But natural philosophy once lost in such a labyrinth, an aberration of the perceptive powers can not fail to follow—at least, in certain domains of nature. As a matter of fact, this fallacious perception promptly made its appearance, and has proved the stumbling-block of science from its earliest days up to the present times. Occultism, mysticism, or whatever the names may be of the various forms of superstition, have sprung from these erroneous conceptions of natural science. It may even be contended that no variety of superstition exists which is