193 NATURE AND MAN. 



be remembered, moreover, that the Ptolemaic system of astronomy, 

 with all its cumbrous ideal mechanism of " centric and excentric, 

 " cycle and epicycle, orb in orb," did intellectually represent all 

 that the astronomer, prior to the invention of the telescope, could 

 see from his actual standpoint, the earth, with an accuracy which 

 was proved by the fulfilment of his predictions. And in that last 

 and most memorable anticipation which has given an imperishable 

 fame to our two illustrious contemporaries, the inadequacy of the 

 basis afforded by actual observation of the perturbations of Uranus, 

 required that it should be supplemented by an assumption of the 

 probable distance of the disturbing planet beyond, which has been 

 shown by subsequent observation to have been only an approxi- 

 mation to the truth. 



Even in this most exact of sciences, therefore, we cannot pro- 

 ceed a step, without translating the actual phenomena of Nature 

 into intellectual representations of those phenomena ; and it is 

 because the Newtonian conception is not only the most simple, 

 but is also, up to the extent of our present knowledge, universal 

 m its conformity to the facts of observation, that we accept it as 

 the only scheme of the universe yet promulgated, which satisfies 

 our intellectual requirements. 



When, under the reign of the Ptolemaic system, any new in- 

 equality was discovered in the motion of a planet, a new wheel 

 had to be added to the ideal mechanism, as Ptolemy said, " to 

 " save appearances." If it should prove, a century hence, that the 

 motion of Neptune himself is disturbed by some other attraction 

 than that exerted by the interior planets, we should confidently 

 expect that not an ideal but a real cause for that disturbance will 

 be found in the existence of another planet beyond. But I trust 

 that I have now made it evident to you, that this confident ex- 

 pectation is not justified by any absolute necessity of Nature, but 

 arises entirely out of our belief in her uniformity ; and into the 

 grounds of this and other primary beliefs, which serve as the 

 foundation of all scientific reasoning, we shall presently inquire. 



There is another class of cases, in which an equal certainty is 

 generally claimed for conclusions that seem to flow immediately 

 from observed facts, though really evolved by intellectual pro- 

 cesses ; the apparent simplicity and directness of those processes 



