Science, true science, which is not intuition but concept, not individuality but universality, cannot be anything but a science of the spirit, that is, of what is universal in reality: Philosophy. If natural _sciences_ be spoken of, apart from philosophy, it is necessary to observe that these are not perfect sciences: they are complexes of knowledge, arbitrarily abstracted and fixed. The so-called natural sciences themselves recognize, in fact, that they are surrounded by limitations. These limitations are nothing more than historical and intuitive data. They calculate, measure, establish equalities, regularity, create classes and types, formulate laws, show in their own way how one fact arises out of other facts; but in their progress they are always met with facts which are known intuitively and historically. Even geometry now states that it rests altogether on hypotheses, since space is not three-dimensional or Euclidean, but this assumption is made use of by preference, because it is more convenient. What there is of truth in the natural sciences, is either philosophy or historical fact. What they contain proper to themselves is abstract and arbitrary. When the natural sciences wish to form themselves into perfect sciences, they must issue from their circle and enter the philosophical circle. This they do when they posit concepts which are anything but natural, such as those of the atom without extension in space, of ether or vibrating matter, of vital force, of space beyond the reach of intuition, and the like. These are true and proper philosophical efforts, 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 assumptions, which cannot be separated from the natural sciences, furthermore explain, not only how, in the progress of knowledge, that which was once considered to be truth descends gradually to the grade of mythological beliefs and imaginary illusions, but also how, among natural scientists, there are some who term all that serves as basis of argument in their teaching _mythical facts, verbal expedients_, or _conventions_. The naturalists and mathematicians who approach the study of the energies of the spirit without preparation, are apt to carry thither these mental habits and to speak, in philosophy, of such and such conventions "as arranged by man." They make conventions of truth and morality, and their supreme convention is the Spirit itself! However, if there are to be conventions, something must exist about which there is no convention to be made, but which is itself the agent of the convention. This is the spiritual activity of man. The limitation of the natural sciences postulates the illimitation of philosophy. [Sidenote] _The phenomenon and the noumenon._