To Teach the History of Science? 55 



Let me give you an example. I trust you will allow me to relate the 

 results of my own experience. I do not choose it because it is my own, 

 but simply because it is the one which I know by far the best. It has 

 been my privilege to teach the history of science in Harvard University 

 for many years, more than thirty, a lifetime. In the course of that long 

 period, I have lectured on almost every aspect and problem of science; 

 I have delivered many hundreds of diflFerent lectures. Some subjects 

 are so important that I have come back to them repeatedly; yet, as at 

 least two years would elapse before I could come back to the same topic 

 and as I was attentive to every novelty concerning it and never stopped 

 gathering new ideas, asking myself new questions, evoking new doubts 

 or solving old ones, when I finally came back to that topic, both the topic 

 and myself were somewhat different; the canvas of my lecture remained 

 perhaps the same, but it was not filled in exactly in the same way. The 

 accent was not put on the same details nor the emphasis in the same 

 places. I am not expressing here vague generalities. As I have gener- 

 ally preserved old lecture notes, I could reconstruct, if it were worth- 

 while, which it is not, the evolution of my views on every important 

 subject, say, Faraday, Darwin, or Pasteur, the discovery of analytical 

 geometry, or of the calculus, the circulation of the blood, or the periodic 

 system. Between one lecture on any one of those subjects and the next, 

 many things might occur, and some of them did occur, for example, the 

 publication of unknown documents, or of a new biography, or a new 

 discovery throwing new light upon the old one, contradicting it, or on 

 the contrary, justifying it, or amplifying it, putting it altogether in a new 

 perspective. It has been truly said of political history that even the 

 best books have no finality; for, on the one hand, new facts are constantly 

 exhumed which may modify our knowledge of the past, even of the most 

 distant past,^^ and on the other hand, we see the past in a different light 

 as our experience increases. The past, as we know it, is not irremediable 

 and final. It could be so only in the eyes of an omniscient god, knowing 

 not only the whole past but the whole future as well. If that be true 

 of political history, it is even more true of the history of science. Think 

 of the theories of light. At the end of last century, the wave theory 

 seemed to be established forever. Crucial experiments had proved its 

 correctness; the electro-magnetic theory had brought a beautiful confir- 

 mation. The judgment of any historian writing at that time would have 

 been different from our own. A similar remark would apply to the his- 

 tory of the periodic system; the introduction of the idea of atomic num- 

 bers threw an entirely new light on it. And to take an earlier example, 

 Galileo's discussion showing that the number of square numbers is as 

 large as the number of positive integers was intriguing,*"^ but it did not 

 assume its full interest until the theory of infinite aggregates had been 



™ Indeed, our knowledge of pre-Hellenic times in the Near East has been deeply 

 modified within our own days. Much of it was entirely unknown before, and the 

 rest is almost entirely renewed or reinterpreted. 



®^ Discor^i e dimostrazioni matematiche intorno a due nuoue scienze (p. 78, Leida 

 1638). 



