The difference between speculative and inductive science is that between fantasy and sound common sense. The latter produces its ideas by means of the outer world, by the help of experience, while fantasy gets its product from the depth of the mind, out of itself. But this method of production is only seemingly one-sided. A thinker can no more think transcendental thoughts which are beyond the reach of experience, than a painter can invent transcendental pictures, transcendental forms. Just as fantasy creates angels by a combination of man and bird, or mermaids by a composition of woman and fish, so all other products of fantasy, though seemingly derived out of itself, are in fact only arbitrarily arranged impressions of the outer world. Reason operates with numbers and orders, time and measures, and other means of experience, while fantasy reproduces the experiences without regard to law and in an arbitrary form. The longing for knowledge has been the cause of speculative attempts to explain the phenomena of life and nature at a time when lack of experience and observation made inductive understanding impossible. Experience was then supplemented by speculation. In later times, when experience had grown, previous speculation was generally recognized as erroneous. But it nevertheless required thousands of years of repeated disappointments on one side and numerous brilliant successes of the inductive method on the other, before these speculative hobbies came into disfavor. Fantasy has certainly a positive power, and speculative intuition, derived from analogy, very often precedes empirical and inductive understanding. But we must remain aware of the fact that so much is assumption and so much actual scientific knowledge. Conscious intuition stimulates scientific research, while pseudo-science closes the door to inductive research. The acquisition of the clear understanding of the distinction between speculation and knowledge is a historical process, the beginning and end of which coincides with the beginning and end of speculative philosophy.