ra. i.] MATTER, MOTION, AND FOECE. 295 



entire sidereal universe, we should find that even the 

 enormous factors already taken into the account would help 

 us but little toward determining the resultant direction 

 in which the man is moving. The comparative ease with 

 which astronomy ascertains the direction of the motions 

 with which it deals, is due to our ability to isolate our- 

 selves theoretically from an indefinitely extended universe 

 of environing bodies ; and this is due to the principle, esta- 

 blished by Galileo, that the relative motions of the parts of 

 an aggregate are not affected by the motion of the whole. 

 If we could include in the problem the entire knowable 

 universe, we should doubtless find the real motions oi a 

 planet as impossible to calculate mathematically as are now 

 the motions of a corpuscle of nerve-substance when thrown 

 out of equilibrium by an act of thinking. 



Nevertheless, because of this principle that the relative 

 motions of parts may be calculated independently of the 

 motion of the whole, we are enabled legitimately to restrict 

 our views, so that motion along the resultant of two or three 

 forces may be determined and predicted with a near ap- 

 proach to accuracy. Witness the ease with which we can 

 calculate the orbit of a comet. But when the forces become 

 more numerous, it becomes impossible to determine their 

 resultant. Witness the excessive difficulty of predicting the 

 direction of currents in the atmosphere. The movements of 

 organisms still more hopelessly baffle our powers of calcula- 

 tion. It is hardly probable that science will ever obtain 

 equations for the motions of a lion in securing his prey ; yet 

 that would be a very shallow philosophy which should seek 

 to assure us that each one of those motions does not take 

 place along the resultant of all the forces involved. To an 

 intelligence sufficiently vast, the motions of the earth in 

 space would doubtless seem as complicated as those of the 

 lion seem to us. But no amount of complexity can alter 

 the fundamental principle that the direction of motion must 



