18 PROGRESS OF HUMAN KNOWLEDGE, 



Kepler, ' Nego ullum motuum perennem non-rectum a 

 Deo conditum esse ' or that dogma of Leibnitz, that 

 nothing in nature occurs per saltum, or without a 

 sufficient reason why it should be so rather than other- 

 wise or Pascal's definition of cases where, though two 

 contraries are severally inconceivable, one must neces- 

 sarily be true or a more specific generalisation in 

 natural history, that there is no living being whose 

 form is geometrically regular, nor any living form 

 having plane faces. . . . Such generalisations, of which 

 many others might be cited, have their value even in 

 the exceptions and contradictions they evoke. The 

 discussion of these often leads to still higher laws, 

 embracing the very exceptions which create the 

 controversy. 



