134 THE GRAMMAR OF SCIENCE 



scientist as a universe of varying motions, motions the why 

 of which is unknown, but the sequences of which are, 

 according to our experience, invariably repeating them- 

 selves. The cause of motion in the scientific sense lying 

 in the sphere of sense-impressions^ cannot be the zvhy of 

 motions, we must seek it in some uniform mitecedent of 

 the motion — such, for example, as the past history of the 

 motion, the relative position of the moving bodies, and so 

 forth. How such antecedents are true scientific causes of 

 motion we shall see in our Chapter VHL devoted to the 

 " Laws of Motion." 



§ 1 1 . — Necessity belongs 1o the World of Conceptions^ 

 not to that of Perccptio7is 



At this point the reader may feel inclined to say: "But 

 surely there is as much necessity that a planet describing 

 its elliptic orbit should at a certain time be in a certain 

 position, as that the angles on the diameter of a circle 

 should be right-angles ? " With this I entirely agree. 

 The theory of planetary motion is in itself as logically 

 necessary as the theory of the circle ; but in both cases 

 the logic and necessity arise from the definitions and 

 axioms with which we mentally start, and do not exist in 

 the sequence of sense-impressions which we hope that they 

 will, at any rate approximately, describe. The necessity 

 lies in the world of conceptions, and is only unconsciously 

 and illogically transferred to the world of perceptions. 



This difference may be well illustrated by an example 

 due to Mr. James Stuart, formerly Professor of Mechanism 

 in Cambridge. Suppose I were to put a stone on a piece 

 of flat ground and walk round it in that particular curve 

 termed an ellipse, which a planet describes about the sun. 

 We will further suppose the stone to be at that particular 

 point termed the focus which in the case of an elliptic 

 orbit is actually occupied by the sun ; and lastly, I will 



^ That the frequently cited " muscular sensation of force " is really only a 

 sense-impression interpreted as one of motion will be shown at a later stage 

 of our work. 



