2/2 THE GRAMMAR OF SCIENCE 



fails to rest in peace until it reaches somewhere the motion 

 of a point, the sizeless ultimate element of matter postulated 

 by Boscovich. We find ourselves again involved in the 

 contradictions which flow from asserting a reality for 

 motion in the phenomenal field. We are again forced to 

 the conclusion that motion is a pure conception, which 

 may describe perceptual changes, but cannot be projected 

 into the phenomenal world without involving us in inex- 

 plicable difficulties. 



§ I 3. — IV/iy do Bodies move ? 



We have left but little space for the discussion of our 

 second question : Why do bodies move ? But the 

 answer to this question must be clear after what precedes. 

 If we mean : Why do sense-impressions change in a 

 certain manner ? — then we have already seen what are 

 the possibilities of knowledge on this point when con- 

 sidering consciousness, the nature of the perceptive faculty 

 and the routine of perceptions (pp. 10 1-7). If we mean : 

 Why do the geometrical symbols by which we concep- 

 tualise material groups of sense-impressions move in a 

 certain fashion ? — then the answer is, that after many 

 guesses we have found these types of motion to be best 

 capable of describing the past and predicting the future 

 routine of our perceptions. If, however, any one persists 

 in phenomenalising our conceptual symbols of motion, 

 then science can only reply to this question : Why does 

 matter move ? We don't know. Let us suppose that 

 the earth actually moves in an ellipse round the sun in a 

 focus, and then let us attempt to analyse the why of it. 

 Well, conceptually we construct this motion out of a 

 certain relative motion of the elementary parts of sun and 

 earth. We say that if these elementary parts have 

 certain relative accelerations when in each other's pre- 

 sence, then the earth will describe an ellipse about the 

 sun. These elementary parts may be looked upon as 

 atoms or groups of atoms, but to save any hypothesis let 

 us simply term them particles of matter. Now, why do 

 two particles when in each other's presence move relative 



