POPULAR SCIENTIFIC LECTU11ES. 



time, pare d priori intuition has "been the 

 anchoring-ground of metaphysicians. It is 

 even more convenient than pure thought, be- 

 cause everything can be heaped on it without 

 going into chains of reasoning, which might 

 bo capable of proof or of refutation The 

 nativistic theory of perception of the senses 

 is the expression of this theory in physiology. 

 All mathematicians united to fight against 

 any attempt to resolve the intuitions into 

 their natural elements ; whether the so-called 

 pure or the empirical, the axioms of geome- 

 try, the principles of mechanics, or the per- 

 cetions of vision. For this reason, there- 

 fore, the mathematical investigations of 

 Lobatschewsky, Gauss, and Eiemann on the 

 alterations which are logically possible in the 

 axioms of geometry ; and the proof that the 

 axiomr are principles which are to be confirm- 

 ed or perhaps even refuted by experience, and 

 can accordingly be acquired from experience 

 these I consider to be very important steps. 

 That all metaphysical sects get into a rage 

 about this must not lead you astray, for these 

 investigations lay the axe at the bases of ap- 

 parently the firmest supports which their 

 claims still possess. Against those investiga- 

 tors who endeavor to eliminate from among 

 the perceptions of the senses, whatever there 

 JZifly be of the actions of memory, and of the 

 repetition of similar impressions, which oc- 

 cur in memory ; whatever, in short, is a mat- 

 ter of experience, against them it is attempt- 

 ed to raise a party cry that they are spiritual- 

 ists. As if memory, experience, and custom 

 were not also facts, whose laws are to be 

 sought, and which are not to be explained 

 away because they cannot be glibly referred 

 to reflex actions, and to the complex of the 

 prolongation of ganglionic cells, and of the 

 connection of nerve-fibres in the brain. 



Indeed, however self-evident, and however 

 important the principle may appear to be, 

 that natural science has to seek for the laws 

 of facts, this principle is nevertheless often 

 forgotten. In recognizing the law found, as 

 a force which rules the processes in nature, 

 we conceive it objectively asa/orce, and such 

 a reference of individual cases to a force 

 which under given conditions produces a 

 definite result, that we designate as a causal 

 explanation of phenomena. We cannot 

 always refer to the forces of atoms ; we speak 

 of a refractive force, of electro-motive and 

 of electro-dynamic force. But do not forget 

 the given conditions and the given result. If 

 these cannot be given, the explanation at- 

 tempted is merely a modest confession of 

 ignorance, and then it is decidedly better to 

 confess this openly. 



If any process in vegetation is referred to 

 forces in the cells, without a closer definition 

 of the conditions among which, and of the 

 direction in which, they work, this can at 

 most assert that the more remote parts of the 

 organism are without influence ; but it would 

 be difficult to confirm this with certainty in 

 more than a few cases. In like manner, the 

 originally definite sense which Johannes 



Muller gave to the rtlea of reflex action, is 

 gradually evaporated into thin, that when an 

 impression has been made on any part of the 

 nervous system, and an action occurs in any 

 other part, this is supposed to have been ex- 

 plained by saying that it is a reflex action. 

 Much may be imposed upon the irr3solvablo 

 complexity of the nerve-fibres of the brain. 

 But the resemblance to the qualitates occultce 

 of ancient medicine is very suspicious. 



From the entire chain of my argument it 

 follows that v/hat I have said against meta- 

 physics is not intended against philosophy. 

 But metaphysicians have always tried to 

 plume themselves on being philosophers, and 

 philosophical amateurs have mostly taken an 

 interest in the high-flying speculations of the 

 metaphysicians, by which they hope in ft 

 short time, and at no great trouble, to learn 

 the whole of what is worth knowing. On 

 another occasion I compared the relation- 

 ship of metaphysics to philosophy with that 

 of astrology to astronomy. The former had 

 the most exciting interest for the public at 

 large, and especially for the fashionable 

 world, and turned its alleged connoisseiirs 

 into influential persons. Astronomy, on the 

 contrary, although it had become the ideal of 

 scientific research, had to be content with a 

 small number of quietly working disciples. 



In like manner, philosophy, if it gives up 

 metaphysics, still possesses a wide and im- 

 portant field, the knowledge of mental and 

 spiritual processes and their laws. Just as 

 the anatomist, when ho has reached the lim- 

 its of microscopic vision, must try to gain 

 an insight into the action of his optical in- 

 strument, in like manner every scientific in- 

 quirer must study minutely the chief instru- 

 ment of his research as to its capabilities. 

 The groping of the medical schools for the 

 last two thousand years is, among other 

 things, an illustration of the harm of errone- 

 ous views in this respect. And the physician, 

 the statesman, the jurist, the clergyman, and 

 the teacher, ought to be able to build upon a 

 knowledge of physical processes if they wish 

 to acquire a true scientific basis for their 

 practical activity. But the true science of 

 philosophy has had, perhaps, to suffer more 

 from the evil mental habits and the false 

 ideals of metaphysics than even medicine 

 itself. 



One word of warning. I should not like 

 you to think that my statements are influ- 

 enced by personal irritation. I need not ex- 

 plain that one who has such opinions as I 

 have laid before j-ou, who impresses on his 

 pupils, whenever he can, the principle that 

 " a metaphysical conclusion is either .1 false 

 conclusion or a concealed experimental con- 

 clusion," that he is not exactly beloved by 

 the votaries of metaphysics or of intuitive 

 conceptions. Metaphysicians, like ail those 

 who cannot give any decisive reasons to their 

 opponents, are usually not very polite in 

 their controversy ; one's own success may 

 approximately be estimated from the increas- 

 ing want of politeness in the replies. 



