THE PREDICTION OF NATURAL PHENOMENA. 545 



the course of time. A remedy for this was sought in complicated ex- 

 tensions of his teachings ; this, however, only made matters so much 

 worse, without explaining the facts. King Alfonso X of Castile is 

 reported to have said to his astronomers that, if the arrangement of 

 the universe had fallen to his lot, he would have made things much 

 more simple. All of these difficulties were suddenly removed by Co- 

 pernicus. The course of the planets now no longer appeared a cause 

 for great perplexity, but admitted of a simple explanation and resolved 

 itself into one grand harmony. Then, after Kepler had discovered the 

 three laws that bear his name and which mark a new era in the science, 

 the mathematical part of the work was brought to an end by Newton's 

 discovery of the law of gravitation. 



Eclipses of the sun and moon always attract general attention. On 

 such occasions it is not only the phenomena themselves that call for 

 our admiration, but mainly the art which makes possible the prediction 

 of these events to the hour, the minute, ay, the second ; by means of 

 which one may know in advance whether the eclipse will be total, par- 

 tial, or annular, what part of the sun or the moon will be first cov- 

 ered, how long the phenomenon will last, and from what parts of the 

 world the eclipse will be visible. 



If a prediction rest on a hypothesis prepared ad hoc, no matter 

 how ingenious it may be, our doubts and our mistrust are justified. 

 In such a case we have the right to ask for confirmation, and to de- 

 mand that the hypothesis shall be extended into a theory by its se- 

 quence, and that this sequence shall stand in accordance with the 

 actual facts. If, however, a prediction is based on a theory which ap- 

 proaches in thoroughness and in extensive confirmation the cases we 

 have cited as examples, and furthermore, if the separate instance be 

 deduced in a strictly logical manner, then the prediction is worthy of 

 our confidence. As most systematic classifications are of value only in 

 affording a general view of the ground, without being able to embrace 

 all cases, so, too, in our classification, we meet with instances of tran- 

 sition and combination. 



Of this, examples are to be found in all the sciences, but notably so 

 in medicine, geology, and meteorology. The predictions in medicine 

 to a great extent form transitions between the first and the second 

 class ; that is to say, they are based in part on Bacon's, in part on Mill's 

 system of induction. Thus, of late, the theory of bacteria has come 

 to be of great importance. This theory is of recent origin, but has 

 deservedly many champions, and offers an insight into new ways, which 

 medicine, in combination with natural science, must explore in order 

 to obtain valuable results. Based on these views, which would desig- 

 nate certain bacteria as carriers of certain diseases, new precaution- 

 ary measures have been adopted, which are to serve as a guard 

 against the phantoms of disease ; and in many places these measures 

 have already proved of great value. 



TOL. XXIX. 35 



