ISAAC NEWTON 85 



collection of natural knowledge, which can only be obtained 

 from outside, from the world as it is, - and it is generally 

 surprising and peculiar in an unexpected way. Without a 

 great store of knowledge thus collected, no great work of 

 science can result, and Newton was fortunate in being in 

 possession of the work of so many rare spirits which we have 

 just reviewed, from Archimedes to Huygens. But the manner 

 in which he grasped this knowledge, and greatly increased 

 it on his own account, was worthy of the great achievement 

 of his predecessors, and forms a monument to them and to 

 him; science should be thankful to possess such a monument. 



Coming now to consider the new matter which he himself 

 added, we are compelled by its richness to limit ourselves 

 somewhat - even more than in the case of Huygens - but not 

 to the point of being unable to give a fairly complete picture. 



One of Newton's especial achievements was his elucidation 

 of the fundamental concepts: mass [massa or corpus)} weight 

 (pondus), force {ms). Without this elucidation, the Principia 

 could not have been written. Mass means for him - and for 

 us to-day - always the measure of inertia, that is, the quan- 

 titative expression of the tendency, discovered by Galileo, 

 of every body to oppose change of velocity. ^ 



1 Newton wrote his Principia in Latin; his Opticks was written in 

 English. 



2 This meaning of the word 'mass' in Newton's work entirely corres- 

 ponds with its application throughout the Principia. The fact that 

 Newton did not expressly define the concept of mass in this way, has 

 often been used as a reproach to him. But we must remember that con- 

 cepts in science do not by any means rest upon the definition of words, 

 but that, on the contrary, a concept must gradually be formed hand in 

 hand with the growth of experience, in order that it may be of such a 

 nature as best to serve the representation of observed reality. When 

 growing experience has brought us to such useful concepts, we may then 

 proceed to describe in words this mental possession, that is to say, to put 

 forward a definition of the concept. What critics may therefore rightly 

 think is the following: that Newton arrived at the complete possession of 

 the concept of mass as we have it to-day, but that he left posterity to 

 produce an improved verbal definition of it, and also to test the usefulness 

 of the concept further. The addition which experience has led us to make 

 to Newton's concept of mass is of quite recent date; it belongs to the last of 

 our series of men of science. It has caused no change in the concept itself. 



