464 Prof. Challis on the Fundamental Ideas of 



by proportional variations of its pressure. The different kinds 

 of physical forces are pressures of the aether acting under differ- 

 ent circumstances, and are regulated by the modes of the mu- 

 tual action of the parts of the fluid. The laws of the physical 

 forces, and of the action of the aether on the atoms of visible 

 and tangible substances, are the proper subjects of mathema- 

 tical research, without which it is not possible to assign reasons 

 for phenomena and the laws of phenomena. 



The above hypotheses are, in part, coincident with those rela- 

 ting to the qualities of bodies contained in Regula III. prefixed 

 to the third book of Newton's Principia; and all of them have 

 been adopted in accordance with rules of philosophy laid down 

 by Newton, viz. (1) to admit no qualities of bodies that are not 

 cognizable and intelligible by sensation and experience ; (2) to 

 frame no arbitrary hypotheses. Where Newton says at the end 

 of the Principia, " hypotheses non fingo," he is referring to spe- 

 culatirc hypotheses not deducible from, or not supported by, 

 phenomena, the same that at the beginning of Regula III. he 

 calls " somnia." But there is nothing in his philosophy opposed 

 to such universal and necessary hypotheses as are the founda- 

 tions of Theory, without which, in fact, theory, regarded as the 

 perception of reasons for phenomena, does not exist. The qua- 

 lities of bodies enumerated in Regula III., such as extension, 

 hardness, mobility, and vis inertia, are hypotheses in the proper 

 sense of the word, inasmuch as they are appropriate foundations 

 of physical theory. It is certainly a remarkable circumstance 

 that Newton has placed at the beginning of his third book, 

 which is mainly devoted to the calculation of the movements of 

 masses acted upon by gravity, a statement of the essential qua- 

 lities of the ultimate constituents of matter, accompanied by the 

 assertion that our perception of these qualities by the seiises is 

 " the foundation of all philosophy." Probably his chief motive 

 in doing this was to distinguish between essential qualities and 

 the attraction of gravity, respecting which he at the same time 

 makes the assertion that he by no means affirms it to be, like 

 vis inertia, essential to bodies, assigning for the distinction the 

 obvious reason that gravity diminishes with distance from the 

 attracting body. In this view Newton has not been followed by 

 modern philosophers. 



Hypotheses that are made the foundations of a physical theory 

 ought, in the first place, to admit of being expressed in terms 

 which personal sensation and experience render perfectly intelli- 

 gible ; secondly, they should be such only as are suggested by 

 observation and experiment ; and thirdly, they must be proper 

 for forming the basis of mathematical reasoning. This last con- 

 dition is a necessary one, because the evidence for the truth of 



