120 THE GRAMMAR OF SCIENCE 



ceptions. The necessity in a law of nature has not the logical 

 must of a geometrical theorem, nor the categorical must of. 

 a human law-giver ; it is merely our experience of a routine, 

 whose stages have neither logical nor volitional order. 



S 4. — Secondary Causes involve no Etiforcement 



Let us endeavour to see a little more closely how the 

 idea of any inherent necessity in the particular order 

 taken by our perceptions disappears from the scientific 

 conception of a sequence of motions — at least from all 

 but the first stage, if the sequence arise from an apparent 

 act of will. Still speaking in the popular sense, we will 

 term the act of will, if it exists, a first cause, and the 

 successive stages of the sequence secondary causes. Our 

 present proposition is that the scientific description of 

 motion involves no idea of enforcement in the successive 

 stages of motion. We shall see in the sequel that the 

 whole tendency of modern physics has been to describe 

 natural phenomena by reducing them to conceptual 

 motions. From these motions we construct the more 

 complex motions by aid of which we describe actual 

 sequences of sense-impressions. But in no single case 

 have we discovered why it is that these motions are 

 taking place ; science describes how they take place, but 

 the why remains a mystery. To term it force might not 

 be so productive of obscurity as it is, were there any 

 suggestion in the elementary text-books that the cause of 

 motion, or of change in motion, may be the structure of 

 the perceptive faculty, or will, or the deity, or any 

 unknowable x amid an unthinkable y and z. The glib 

 transition from force as a cause to force as a measure of 

 motion too often screens the ignorance which it is as 

 much the duty of science to proclaim from the house- 

 tops as it is its duty to assert knowledge on other points. 

 Primitive man placed a sun-god behind the sun (as some 

 of us still place a storm-god behind the storm), because 

 he did not see how and why it moved. The physicist 

 now proceeds to describe how the sun moves, by describ- 



