198 THE PRINCIPLES OF SCIENCE 



only to manifest, that the air hath a spring, and to relate some of its 

 effects. 



In their time these passages expressed a novel and important idea: 

 the value of even fragmentary knowledge, i.e., isolated laws. Both 

 authors also imply that a search for more complete understanding 

 then seemed unpropitious, but neither implies that such understand- 

 ing is unattainable. As a matter of fact, Newton did of course find 

 causes for the acceleration of falling bodies and the springiness of 

 air. We construe the second case in a very different manner, but the 

 "adequate cause" is for us, as for Newton, an explanation in terms of 

 forces. 



What of the celebrated Newtonian disclaimer: hypotheses non 

 fingo? What Newton actually says is this : 



But hitherto I have not been able to discover the cause of those prop- 

 erties of gravity from phenomena, and I feign no hypotheses; ... to 

 us it is enough that gravity does really exist, and act according to the 

 laws which we have explained, and abundantly sei-ves to account for 

 all the motions of the celestial bodies, and of our sea. 



The word "hitherto" is illuminating. Like Galileo and Boyle, Newton 

 wisely declines speculation when, for the purpose in hand, he felt the 

 need as clearly insufficient as the evidence. But this is a renunciation 

 neither final nor complete. Far from it! In the very next (concluding) 

 paragraph of the scholium in which the above passage appears, New- 

 ton considers certain possible causes of gravity. He remained pas- 

 sionately interested in the problem, and never despaired of solving 

 it. Certain speculations offered in the later editions of the Opticks 

 left him still unsatisfied, but his feelings remain apparent in the 

 words I have italicized in the following famous passage. 



To tell us that every species of things is endowed with an occult spe- 

 cific quality by which it acts and produces manifest effects, is to tell 

 us nothing: but to derive two or three general principles of motion 

 from phenomena, and afterwards to tell us how the properties and 

 actions of all corporeal things follow from those manifest principles, 

 would be a veiy great step in philosophy, though the causes of those 

 principles were not yet discovered; and therefore I scruple not to pro- 

 pose the principles of motion above mentioned, they being of very 

 general extent, and leave their causes to be found out. 



