Science, true science, which is not intuition but concept, not individuality but universality, cannot be anything but science of the spirit, that is, of what reality has of universal: Philosophy. If natural _sciences_ be spoken of, apart from philosophy, we must observe that these are not perfect sciences: they are aggregates of cognitions, arbitrarily abstracted and fixed. The so-called natural sciences indeed themselves recognize that they are surrounded by limitations, and these limitations are nothing but historical and intuitive data. They calculate, measure, establish equalities and uniformities, create classes and types, formulate laws, show in their own way how one fact arises out of other facts; but while doing this they are constantly running into facts known intuitively and historically. Even geometry now states that it rests altogether on hypotheses, since threedimensional or Euclidean space is but one of the possible spaces, selected for purposes of study because more convenient. What is true in the natural sciences is either philosophy or historical fact. What of properly naturalistic they contain, is abstraction and caprice. When the natural sciences wish to become perfect sciences, they must leave their circle and enter philosophy. They do this when they posit concepts which are anything but naturalistic, such as those of the unextended atom, of ether or vibration, of vital force, of non-intuitional space, and the like. These are true and proper attempts at philosophy, when they are not mere words void of meaning. The concepts of natural science are, without doubt, most useful; but one cannot obtain from them that _system_ which belongs only to the spirit. These historical and intuitive data which cannot be eliminated from the natural sciences furthermore explain not only how, with the advance of knowledge, what was once believed to be true sinks gradually to the level of mythological belief and fantastic illusion, but also how among natural scientists some are to be found who call everything in their sciences upon which reasoning is founded _mythical facts, verbal expedients,_ or _conventions._ Natural scientists and mathematicians who approach the study of the energies of the spirit without preparation, are apt to carry thither such mental habits and to speak in philosophy of such and such conventions as "decreed by man." They make conventions of truth and morality, and a supreme convention of the Spirit itself! But if there are to be conventions, something must exist which is no convention, but is itself the author of conventions. This is the spiritual activity of man. The limitation of the natural sciences postulates the illimitability of philosophy. [Sidenote: _The phenomenon and the noumenon._]