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all. Space is symmetrical yet the laws of motion would present 

 no symmetry. They should be able to distinguish between right and 

 left. They would see, for instance, that cyclones always turn in the 

 same direction, while for reasons of symmetry they should turn in- 

 differently in any direction. If our scientists were able by dint of 

 much hard work to make their universe perfectly symmetrical, this 

 symmetry would not subsist, although there is no apparent reason 

 why it should be disturbed in one direction more than in another. 

 They would extract this from the situation no doubt they would 

 invent something which would not be more extraordinary than the 

 glass spheres of Ptolemy, and would thus go on accumulating compli- 

 cations until the long-expected Copernicus would sweep them all 

 away with a single blow, saying it is much more simple to admit that 

 the earth turns round. Just as our Copernicus said to us : " It is 

 more convenient to suppose that the earth turns round, because the 

 laws of astronomy are thus expressed in a more simple language," so 

 he would say to them : " It is more convenient to suppose that the 

 earth turns round, because the laws of mechanics are thus expressed in 

 much more simple language." That does not prevent absolute space 

 that is to say, the point to which we must refer the earth to know 

 if it really does turn round from having no objective existence. 

 And hence this affirmation : " the earth turns round," has no mean- 

 ing, since it cannot be verified by experiment; since such an experi- 

 ment not only cannot be realized or even dreamed of by the most 

 daring Jules Verne, but cannot even be conceived of without contra- 

 diction ; or, in other words, these two propositions, " the earth turns 

 round," and " it is more convenient to suppose that the earth turns 

 round," have one and the same meaning. There is nothing more in 

 one than in the other. Perhaps they will not be content with this, 

 and may find it surprising that among all the hypotheses, or rather 

 all the conventions, that can be made on this subject there is one 

 which is more convenient than the rest. But if we have admitted it 

 without difficulty when it is a question of the laws of astronomy, why 

 should we object when it is a question of the laws of mechanics? We 

 have seen that the co-ordinates of bodies are determined by differ- 

 ential equations of the second order, and that so are the differences 

 of these co-ordinates. This is what we have called the generalized 

 principle of inertia, and the principle of relative motion. If the dis- 

 tances of these bodies were determined in the same way by equations 

 of the second order, it seems that the mind should be entirely satisfied. 

 How far does the mind receive this satisfaction, and why is it not 

 content with it ? To explain this we had better take a simple example. 

 I assume a system analogous to our solar system, but in which fixed 

 stars foreign to this system cannot be perceived, so that astronomers 

 can only observe the mutual distances of planets and the sun, and not 



