212 INTRODUCTION TO PHILOSOPHY OF SCIENCE 



shock last century with the formulation of non-Euclidean 

 geometries, and physics passed through a similar crisis with 

 the advent of the theory of relativity. Descriptive science is 

 able to make readjustments by specific modifications in the 

 region where the error is discovered. But in such explanatory 

 science there are no isolated regions; every element has 

 determinate relations to every other element. An explanatory 

 science is organic in the same sense that the human body is; a 

 purely localized disturbance may "make one sick all over." 

 (2) An explanatory science explains. Here, again, as in 

 the case of descriptive science, this statement may be a pure 

 tautology unless one interprets it. Explanatory science 

 answers the question "Why?" and thus eliminates all brute 

 facts. A brute fact is one which is unrelated, and might 

 equally well have been otherwise. There are no such facts 

 in a highly developed explanatory science. For every de- 

 scriptive proposition is derivable from an hypothesis, and 

 if it were otherwise the hypothesis would of necessity be 

 otherwise; but if the hypothesis were otherwise all other 

 descriptive propositions deducible from it would be different. 

 Hence no descriptive proposition can any longer be said to 

 assert a brute fact, for it has found its place in a general 

 system of propositions which represents a general system of 

 facts. Every descriptive proposition in the scheme has two 

 definite relations — one extensional, to the events which it 

 describes, and the other intensional, to the hypothesis from 

 which it can be deduced. The extensional relation verifies 

 the proposition, and the intensional relation explains the 

 proposition. For example, the proposition "This metal bar 

 expands when heated' is verified by a simple experiment 

 which produces the event described in the proposition. The 

 proposition in question is explained when it is shown that 

 it can be deduced from a general theory stating that the bar 

 is made up of molecules which are put into rapid motion 

 when the bar is heated, and which tend by the violence of 

 their motion to separate one from another and thus to in- 

 crease the space which is occupied by their aggregate. 



