of force (Robert Mayer, 1842). This monistic association of the two fundamental laws, and establishment of the unified law of substance, has met with a good deal of agreement, but also with some opposition; but the most violent attacks were directed against my monistic theory of knowledge, or against the method I followed in seeking to solve the riddle of the universe. The only paths which I had recognized as profitable were those of experience and thought—or empirical knowledge and speculation. I had insisted that these two methods supplemented each other, and that they alone, under the direction of reason, lead to the attainment of truth. At the same time I had rejected as false two other much-frequented paths which purported to lead directly to a profounder knowledge, the ways of emotion and revelation; both of these are in opposition to reason, since they demand a belief in miracles.
"All natural science is philosophy, and all true philosophy is natural science. All true science is natural philosophy." I expressed in these words the general result of my monistic studies in 1866 (in the twenty-seventh chapter of my Generelle Morphologie). I then laid it down as the fundamental principle of the monistic system that the unity of nature and the unity of science follow absolutely from any connected study of modern philosophic science, and I expressed my conviction in these terms: "All human science is knowledge based on experience, or empirical philosophy; or, if the title be preferred, philosophic empiricism. Thoughtful experience, or thought based on experience, is the only way and method to be followed in the search for truth." I endeavored to establish these theses conclusively in the first book of the Generelle Morphologie, which contains (p. 108) a critical and methodological introduction to this science. Not only are those methods considered "which must necessarily supplement each other" (I. Empiricism and Philosophy; II. Analysis and Synthesis; III. Induction and Deduction), but also those "which necessarily exclude each other" (IV. Dogmatism and Criticism; V. Teleology and Causality, or Vitalism and Mechanicism; VI. Dualism and Monism). The monistic principles which I developed there thirty-eight years ago have only been confirmed by my subsequent labors, and so I may refer the interested reader to that work. The Riddle of the Universe is in the main an attempt to introduce to the general reader in a convenient form the chief points of the monistic system I established. However, the opposition which has been aroused by the general philosophic observations of the Riddle compels me to give a further explanation of the chief features of my theory of knowledge.
All true science that deserves the name is based on a collection of experiences, and consists of conclusions that have been reached by a rational connection of these experiences. "Only in experience is there truth," says Kant. The external world is the object that acts on man's organs of sense, and in the internal sense-centres of the cortex of the brain these impressions are subjectively transformed into presentations. The thought-centres, or association centres, of the cortex (whether or no one distinguishes them from the sense-centres) are the real organs of the mind that unite these presentations into conclusions. The two methods of forming these conclusions—induction and deduction, the formation of arguments and concepts, thought and consciousness—make up together the cerebral function we call reason. These long familiar and fundamental truths, the recognition of which I have described for thirty-eight years as the first condition for solving the riddle of life, are still far from being generally appreciated. On the contrary, we find them combated by the extreme representatives of both tendencies of science. On the one side, the empirical and descriptive school would reduce the whole task to experience, without calling in the aid of philosophy; while philosophic speculation, on the other side, would dispense with experience and endeavor to construct the world by pure thought.
Starting from the correct principle that all science originally has its source in experience, the representatives of "experimental science" affirm that their task consists solely in the exact observation of "facts" and the classification and description of them, and that philosophic speculation is nothing more than an idle play of ideas. Hence this one-sided sensualism, as Condillac and Hume especially maintained it, affirmed that the whole action of the mind consists in a manipulation of sense-impressions. This narrow empirical conception spread very widely during the nineteenth century, particularly in the second half, among the rapidly advancing sciences; it was favored by the specialism which grew up in the necessary division of labor. The majority of scientists are still of opinion that their task is confined to the exact observation and description of facts. All that goes beyond this, and especially all far-reaching philosophic conclusions from their accumulated observations, are regarded by them with suspicion. Rudolph Virchow strongly emphasized this narrow empirical tendency ten years ago. In his speech on the foundation of the Berlin University he explained the "transition from the philosophic to the scientific age"; he said that the sole aim of science is "the knowledge of facts, the objective investigation of natural phenomena in detail." The former politician seemed to forget that he had maintained a precisely opposite view forty years before (at Würtzburg), and that his own great achievement, the creation of cellular pathology, was a philosophic construction—the formation of a new and comprehensive theory of disease by the combination of countless observations and the conclusions deduced therefrom.
No science of any kind whatever consists solely in the description of observed facts. Hence we can only regard it as a pitiful contradiction in terms when we find biology classed in official documents to-day as a "descriptive science," and physics opposed to it as an "explanatory science." As if in both cases we had not, after describing the observed phenomena, to pass on to trace them to their causes—that is, to explain them—by means of rational inferences! But it is even more regrettable to find that one of the ablest scientists of Germany, Gustav Kirchhoff, has claimed that description is the final and the highest task of science. The famous discoverer of spectrum analysis says in his Lectures on Mathematical Physics and Mechanics (1877): "It is the work of science to describe the movements perceived in Nature, in the most complete and simplest fashion." There is no meaning in this statement unless we take the word "description" in a quite unusual sense—unless "complete description" is meant to include explanation. For thousands of years true science has been, not merely a simple description of individual facts, but an explanation of them by tracing them to their causes. It is true that our knowledge of them is always imperfect, or even hypothetical; but this is equally true of the description of facts. Kirchhoff's statement is in flagrant contradiction to his own great achievement, the founding of spectrum analysis; for the extraordinary significance of this does not lie in the discovery of the wonderful facts of spectroscopic optics and the "complete description" of individual spectra, but in the rational grouping and interpretation of them. The far-reaching conclusions that he has drawn from them have opened out entirely new paths to physics and chemistry. Hence Kirchhoff is in as sad a plight as Virchow when he formulates so precarious a principle. However, these statements of the two great scientists have done a great deal of harm, as they have widened still more the deep gulf between science and philosophy. It may be of some service if a few thousand of the thoughtless followers of "descriptive science" are persuaded to refrain from attempts at explanation of facts. But the master-builders of science cannot be content with the collection of dead material; they must press on to the knowledge of causes by a rational manipulation of their facts.
The accurate and discriminating observation of facts, supported by careful experiment, is certainly a great advantage that modern science has over all earlier efforts to attain the truth. The distinguished thinkers of classic antiquity were far superior to most modern scientists and philosophers in regard to judgment and reasoning, or all the subtler processes of thought; but they were superficial and unpractised observers, and were barely acquainted with experiment. In the Middle Ages scientific work degenerated in both its aspects, as the dominant creed demanded only faith and the recognition of its supernatural revelation, and depreciated observation. The great importance of this as a foundation of real knowledge was first appreciated by Bacon of Verulam, whose Novum Organon (1620) laid down the principles of scientific knowledge, in opposition to the current scholasticism derived from Aristotle and his Organon. Bacon became the founder of modern empirical investigation, not only by making careful and exact observation of phenomena the basis of all philosophy, but also in demanding the supplementing of this by experiment; by this experiment he understood the putting of a question to Nature, as it were, which she must herself answer—a kind of observation under definite and deliberate conditions.
This more rigorous method of