By this empirical method we find first of all that philosophy is originally not a specialized science working with other sciences, but a generic name for all knowledge, the essence of all science, just as art is the essence of the various arts. Whoever made knowledge, whoever made brain work his essential occupation, every thinker without regard to the contents of his thoughts, was originally a philosopher.
But when with the progressive increase of human knowledge, the various departments detached themselves from the mother of all wisdom, especially since the origin of natural sciences, philosophy became known, not so much by its content as by its form. All other sciences are distinguished by their various objects, while philosophy is marked by its own method. Of course, it also has its object and purpose. It desires to understand the universal whole, the cosmos. But it is not this object, this aim, by which philosophy is characterized; it is rather the manner in which this object is accomplished.
All other sciences occupy themselves with special things, and if they consider the universe at all, they do so only in its bearing on the special objects of their study, the parts of which the universe is composed. Alexander von Humboldt says in his introduction to his "Cosmos" that he is limiting himself to an empirical consideration, to a physical research, which seeks to elucidate the uniformity and unity by means of the great variety. And all inductive sciences arrive at general conclusions and conceptions only by way of their occupation with special and concrete things. For this reason they claim that their conclusions are based on facts.
Speculative philosophy proceeds by the opposite method. Thought, the object of its study, may be some special question, yet it does not follow this up in the concrete. It rejects as fallacious the evidence of the senses, the physical experience gained by means of the eye and ear, hand and brain, and limits itself to "pure" and absolutely abstract thought, in order to understand thus by the unit of human reason the multiplicity of the universe. In seeking for an answer to the question: What is philosophy? which question we are specially discussing just now, speculative philosophy would not start out from its actual material form, from its wooden and pigskin volumes, from its great and small essays, in order to arrive at a conception of its object. On the contrary, the speculative philosopher turns to introspection and looks in the depths of his own mind for the true concept of philosophy. And by this standard he separates the impression of his senses into true or erroneous. This speculative method has hardly ever dealt in tangible things, unless we recognize this philosophical method in every unscientific concept of nature which populated the world with spooks. The rudiments of scientific speculation occasionally dealt with the course of the sun and the globe. But since inductive astronomy cultivates these fields with greater success, speculative philosophy limits itself entirely to abstract discussions. And in this line of research as well as in all others it is characterized by the production of its results out of the idea or the concept.
For empirical science, for the inductive method, the multiplicity of experiences is the first basis, and thought the second. Speculative philosophy, on the other hand, seeks to arrive at scientific truth without the help of experience. It rejects the socalled transient facts as a foundation of philosophical understanding, and declares that it should be absolute, exalted above time and space. Speculative philosophy does not wish to be scientific physics, but metaphysics. It regards it as its task to find by "pure" reason, and without the assistance of experience, a system, a logic, or a theory of science, by which everything worth knowing is supposed to be reeled off logically and systematically, in about the same way in which we derive grammatically the various forms of a word from its root. But the physical sciences operate on the assumption that our faculty of understanding, to use a familiar illustration, resembles a piece of soft wax which receives impressions from outside, or a clean slate on which experience writes its lines. Speculative philosophy, on the other hand, assumes that certain ideas are innate and may be dipped and produced from the depths of the mind by means of thought.
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.
In ancient times, common sense operated in common with fantasy, the inductive with the speculative method. The discussion of their differences begins only with the understanding of the numerous disappointments caused by the still inexperienced judgment which have prevented an unobstructed view of the question up to modern times. But instead of attributing these disappointments to lack of understanding, they were charged to the account of the imperfection of the senses. The senses were called impostors and material phenomena untrue images. Who has not heard the lament about the unreliability of the senses? The misunderstanding of nature and of its phenomena led to a serious rupture with sense perceptions. The philosophers had deceived themselves and thought they had been deceived by the senses. In their anger they turned disdainfully away from the world of sensations. With the same uncritical faith with which the semblance had hitherto been accepted as truth, now uncritical doubt rejected the truth of sensations altogether. Research abandoned nature and experience, and began the work of speculative philosophy by "pure" thought.
But no! Science did not permit itself to be entirely led astray from the path of common sense, from the way of truth of sense perceptions. Natural science soon stepped into the breach, and its brilliant successes gained for the inductive method the consciousness of its fertility, while on the other hand philosophy searched for a system by which all the great general truths might be opened up without specialized study, without sense perception and observation, by mere reason alone.