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SCIENCE 



[N. S. Vol. XXXIX. No. 1007 



jects are: (1) Historical objects, (2) mech- 

 anisms, and (3) statistically defined as- 

 semblages. The three sorts of methods 

 are: The historical, the mechanical and the 

 statistical. 



Clerk Maxwell's few but momentous ob- 

 servations upon these three fields of scien- 

 tific knowledge have a beautiful brevity, 

 and show a fairly poetical skill of imagi- 

 nation whereby he finds and expresses his 

 illustrations both of scientific ideas and of 

 methods. I can not follow the master in 

 his own skill. And I shall be unable to use 

 his language. I must portray his classifi- 

 cation in my own way, and must use my 

 own illustrations. 



If you wish to come into closer touch 

 with this aspect of the master's thought, 

 you may use the concluding passage of his 

 famous elementary treatise on the ' ' Theory 

 of Heat, ' ' and several remarks in his article 

 on "Atoms" in the ninth edition of the 

 Encyclopedia Britannica. In addition I 

 may refer you to the citations made by 

 Theodore Merz in the eleventh chapter of 

 Volume II. of his "History of European 

 Thought in the Nineteenth Century" (pp. 

 599, 601 and 603). 



Let me briefly review, with a few illus- 

 trations, this classification of the three 

 fields and the three methods of natural 

 science. 



Science deals either with substantial things 

 (such as atoms or organisms) or else with 

 events. Let us confine ourselves here to 

 the works of science in its dealings with 

 natural events and processes. Science deals 

 with the historical when its objects are in- 

 dividual events or complexes of events, 

 such as is a single solar eclipse, or such as 

 is the birth or the death of this man, or the 

 performance of just this act of choice by 

 this individual voluntary agent. 



Science deals with the mechanical when 

 its objects are the invariant laws to which 



all the individual events of some field of 

 inquiry are subject, and when such in- 

 variant laws actually exist, and can be 

 used to compute and to predict actual 

 events. Thus, if the acceleration which 

 every individual body belonging to a sys- 

 tem of material bodies undergoes depends 

 at every instant, in an invariant way, upon 

 the spatial configuration of the system of 

 bodies at just that moment, the system is a 

 mechanical system — such, for instance, as 

 a system of bodies moving in accordance 

 with the Newtonian law of gravitation. 



Science deals, in the third place, with the 

 statistical, when it studies the averages in 

 terms of which aggregates or collections of 

 events can be characterized, and when it 

 considers not the invariant laws, but the 

 always variable possibilities that these 

 averages will be subject to certain uniform- 

 ities, and will undergo definable changes. 



In brief, the object of historical knowl- 

 edge is the single event, occurring, in the 

 ideally simple case, to an individual 

 thing. A free-will act or an observed 

 eclipse serves as an example. The object 

 of mechanical knowledge is the unchang- 

 ing natural law under which every event 

 of some type can be subsumed. Sometimes 

 the object of mechanical science may be an 

 individual event, but only in so far as, like 

 the eclipse, it can be predicted by means of 

 such an invariant law. The object of sta- 

 tistical knowledge is not the single event 

 and is not the invariant law, but is the 

 relatively uniform behavior of some aver- 

 age constitution, belonging to an aggregate 

 of things and events, and the probability 

 that this average behavior will remain, 

 within limits, approximately, although al- 

 ways imperfectly uniform. 



VI. APPLICATIONS OP THIS CLASSIFICATION 



In view of this classification of the ob- 

 jects of scientific knowledge, you may see 



