DEMONSTRATION, AND NECESSARY TRUTHS. 157 



lisbed and familiar experience ; or even to old and familiar habits of 

 thought. And this difficulty is a necessary result of the fundamental 

 laws of the human mind. When we have often seen and thought of 

 two things together, and have never in any one instance either seen 

 or thought of them separately, there is by the primary laws of asso- 

 ciation an increasing difficulty, which in the end becomes insuperable, 

 of conceiving the two things apart. This is most of all conspicuous 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 accustomed 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 man of the most 

 practised intellect is not exempt from the universal laws of our concep- 

 tive faculty. If daily habit presents to him for a long period two facts in 

 combination, and if he is not led during that period either by accident 

 or intention to think of them apart, he will in time become incapable 

 of doing so even by the strongest effiart ; 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 wisest men rejected as impossible, because inconceivable, 

 things which their posterity, by earlier practice and longer perseve- 

 rance in the attempt, found it quite easy to conceive, and which every- 

 body 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 gravitation of all bodies towards one 

 another, on the faith of a general proposition, the reverse of which 

 seemed to them to be inconceivable — the proposition that a body can- 

 not act where it is not. All the cumbrous machinery of imaginary 

 vortices, assumed without the smallest particle of evidence, appeared 

 to these philosophers a more rational mode of explaining the heavenly 

 motions, than one which involved what seemed to them so great an 

 absurdity.* And they no doubt found it as impossible to conceive 

 that a body should act upon the earth, at the distance of the sun or 

 moon, as we find it to conceive an end to space or time, or two straight 

 lines inclosing a space. Newton himself had not been able to realize 

 the conception, or we should not have had his hypothesis of a subtle 



* It would be difficult to name a man more remarkable at once for the greatness and the 

 universality of his intellectual powers, than Leibnitz. Yet this great man gave as a reaspn 

 for rejecting Newton's scheme of the solar system, that God could not make a body revolve 

 rouiid a distant centre, unless either by some impelling mechanism, or by miracle : — " Tout- 

 ce qui n'est pas explicable," says he in a letter to the Abbe Conti, "par la nature des crea- 

 tures, est miraculeux. II no suffit pas de dire: Dieu a fait une telle loi de nature : done 

 la chose est naturelle. II faut que la loi soit executable par les natures des creatures. Si 

 Dieu donnait cette loi, par exemple, a un corps libre, de toumer a I'entour d'un certain 

 centre, il faudrait ou qu'il y joignil d'aulres corps q-ui par Icur impulsion Vobligeasscnt de rcster 

 toujours dans son orbite circidaire, ou qu'il nut un ange a ses trousses, ou enfm il faudrait qu'il y 

 concourut extraordinairement ; car naturellement il s'ecartera par la tangente." — Worlui iff 

 Leibnitz, ed. Dutens, iii., 4-lC. 



