DEMONSTRATION, AND NECESSARY TRUTHS. 267 



separately, there is by the primary law of association an in 

 creasing difficulty, which may in the end become insuperable, 

 of conceiving the two things apart. This is most of all con 

 spicuous in uneducated persons, who are in general utterly 

 unable to separate any two ideas which have once become 

 firmly associated in their minds ; and if persons of cultivated 

 intellect have any advantage on the point, it is only because, 

 having seen and heard and read more, and being more accus 

 tomed to exercise their imagination, they have experienced 

 their sensations and thoughts in more varied combinations, and 

 have been prevented from forming many of these inseparable 

 associations. But this advantage has necessarily its limits. 

 The most practised intellect is not exempt from the universal 

 laws of our conceptive faculty. If daily habit presents to 

 any one for a long period two facts in combination, and if he 

 is not led during that period either by accident or by his 

 voluntary mental operations to think of them apart, he will 

 probably in time become incapable of doing so even by the 

 strongest effort ; and the supposition that the two facts can be 

 separated in nature, will at last present itself to his mind 

 with all the characters of an inconceivable phenomenon.* 

 There are remarkable instances of this in the history of science : 

 instances in which the most instructed men rejected as impos 

 sible, because inconceivable, things which their posterity, by 

 earlier practice and longer perseverance in the attempt, found 

 it quite easy to conceive, and which everybody now knows to 

 be true. There was a time when men of the most cultivated 

 intellects, and the most emancipated from the dominion of 

 early prejudice, could not credit the existence of antipodes ; 

 were unable to conceive, in opposition to old association, the 

 force of gravity acting upwards instead of downwards. The 

 Cartesians long rejected the Newtonian doctrine of the gravi- 



b &quot;If all mankind had spoken one language, we cannot doubt that there 

 would have been a powerful, perhaps a universal, school of philosophers, who 

 would have believed in the inherent connexion between names and things, who 

 would have taken the sound man to be the mode of agitating the air which is 

 essentially communicative of the ideas of reason, cookery, bipedality, &c.&quot; De 

 Morgan, Formal Logic, p. 246. 



