It is a greater and a commoner error that the phenomena which we best understand are those which are of most frequent occurrence, and which are most universal and simple; for, on the contrary, these are just the phenomena that we are most accustomed to see about us, and to be ignorant of. It is just as inexplicable to us that a stone should fall to the earth as that an animal should move itself. It has been supposed, as we have remarked above, that, starting from the most universal forces of nature (gravitation, cohesion, impenetrability), it was possible to explain from them the rarer forces, which only operate under a combination of circumstances (for example, chemical quality, electricity, magnetism), and, lastly, from these to understand the organism and the life of animals, and even the nature of human knowing and willing. Men resigned themselves without a word to starting from mere _qualitates occultæ_, the elucidation of which was entirely given up, for they intended to build upon them, not to investigate them. Such an intention cannot, as we have already said, be carried out. But apart from this, such structures would always stand in the air. What is the use of explanations which ultimately refer us to something which is quite as unknown as the problem with which we started? Do we in the end understand more of the inner nature of these universal natural forces than of the inner nature of an animal? Is not the one as much a sealed book to us as the other? Unfathomable because it is without ground, because it is the content, that which the phenomenon is, and which can never be referred to the form, to the how, to the principle of sufficient reason. But we, who have in view not etiology but philosophy, that is, not relative but unconditioned knowledge of the real nature of the world, take the opposite course, and start from that which is immediately and most completely known to us, and fully and entirely trusted by us—that which lies nearest to us, in order to understand that which is known to us only at a distance, one-sidedly and indirectly. From the most powerful, most significant, and most distinct phenomenon we seek to arrive at an understanding of those that are less complete and weaker. With the exception of my own body, all things are known to me only on _one_ side, that of the idea. Their inner nature remains hidden from me and a profound secret, even if I know all the causes from which their changes follow. Only by comparison with that which goes on in me if my body performs an action when I am influenced by a motive—only by comparison, I say, with what is the inner nature of my own changes determined by external reasons, can I obtain insight into the way in which these lifeless bodies change under the influence of causes, and so understand what is their inner nature. For the knowledge of the causes of the manifestation of this inner nature affords me merely the rule of its appearance in time and space, and nothing more.