86 THE GRAMMAR OF SCIENCE 



sequences of sense-impressions, they may be facts to be 

 dealt with scientifically, but they are not laws in them- 

 selves, at least not in any useful interpretation of the 

 word. The changes of the whole planetary system might 

 be perceived, and even those perceptions translated into 

 words with a fulness surpassing that of our most accurate 

 modern observer, and yet neither the sequence of per- 

 ceptions in itself nor the description involve the existence 

 of any law. The sequence of perceptions has to be 

 compared with other sequences, classification and general- 

 isation have to follow; conceptions and ideas, pure products 

 of the mind, must be formed, before a description can be 

 given of a range of sequences which, by its conciseness 

 and comprehensiveness, is worthy of the name of scientific 

 law. 



Let it be noted that in this it is not only the process 

 of reaching scientific law which is mental, but that the 

 law itself when reached involves an association of natural 

 facts or phenomena with mental conceptions, lying quite 

 outside the particular field of those phenomena. Without 

 the mental conceptions the law could not be, and it only 

 comes into existence when these mental conceptions are 

 first associated with the phenomena. The law of gravita- 

 tion is not so much the discovery by Newton of a rule 

 guiding the motion of the planets as his invention of a 

 method of briefly describing the sequences of sense- 

 impressions, which we term planetary motion. He did 

 this in terms of a purely mental conception, namely, 

 mutual acceleration.^ Newton first brought the idea of 

 mutual acceleration of a certain type into association with 

 a certain range of phenomena, and was thus enabled to 

 state a formula, which, by what we may term mental 

 shorthand, resumes a vast number of observed sequences. 

 The statement of this formula was not so much the 

 discovery as the creation of the law of gravitation. We 

 are thus to understand by a law in science, i.e. by a " law 

 of nature," a resume in mental shorthand, which replaces 



1 The reader will find mutual acceleration fully defined and discussed in 

 Chapter VIII. 



