A HISTORY OF SCIENCE 



theory, and the feeble retorts of his conservative 

 antagonist : 



" Salviati. Let us then begin our discussion with the 

 consideration that, whatever motion may be attrib- 

 uted to the earth, yet we, as dwellers upon it, and hence 

 as participators in its motion, cannot possibly perceive 

 anything of it, presupposing that we are to consider 

 only earthly things. On the other hand, it is just 

 as necessary that this same motion belong apparently 

 to all other bodies and visible objects, which, being 

 separated from the earth, do not take part in its mo- 

 tion. The correct method to discover whether one can 

 ascribe motion to the earth, and what kind of motion, 

 is, therefore, to investigate and observe whether in 

 bodies outside the earth a perceptible motion may 

 be discovered which belongs to all alike. Because a 

 movement which is perceptible only in the moon, for 

 instance, and has nothing to do with Venus or Jupiter 

 or other stars, cannot possibly be peculiar to the earth, 

 nor can its seat be anywhere else than in the moon. 

 Now there is one such universal movement which con- 

 trols all others namely, that which the sun, moon, 

 the other planets, the fixed stars in short, the whole 

 universe, with the single exception of the earth appears 

 to execute from east to west in the space of twenty- 

 four hours. This now, as it appears at the first glance 

 anyway, might just as well be a motion of the earth 

 alone as of all the rest of the universe with the excep- 

 tion of the earth, for the same phenomena would re- 

 sult from either hypothesis. Beginning with the most 

 general, I will enumerate the reasons which seem to 



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