84 THE GRAMMAR OF SCIENCE 



being, whose attributes are still as essentially finite as our 

 own, would be able to do what is at present impossible to us. 

 For we have seen that the molecules in a vessel of air at 

 uniform temperature are moving with velocities by no 

 means uniform, though the mean velocity of any great 

 number of them, arbitrarily selected, is almost exactly 

 uniform. Now let us suppose that such a vessel is divided 

 into two portions, A and B, by a division in which there 

 is a small hole, and that a being,^ who can see the 

 individual molecules, opens and closes this hole, so as to 

 allow only the swifter molecules to pass from A to B, and 

 only the slower ones to pass from B to A. He will thus, 

 without expenditure of work, raise the temperature of B 

 and lower that of A, in contradiction to the second law of 

 thermo-dynamics." 



To render this passage clear to the lay reader, we have 

 only to add that in this kinetic theory the temperature of 

 a gas depends upon the mean speed of its molecules. 

 Now the Second Law of thermo-dynamics resumes with 

 undoubted correctness a wide range of human experience, 

 and is, to that extent, as much a law of nature as that of 

 gravitation. But the kinetic theory of gases, whether it 

 be hypothetical or not, enables us to conceive a demon 

 having a perceptive faculty differing rather in degree than 

 quality from our own, for whom the Second Law of 

 thermo-dynamics would not necessarily be a law of nature. 

 Such a conception enables us to grasp how relative what 

 we term nature is to the faculty which perceives it. 

 Scientific law does not, any more than sense-impression, 

 lie in a universe outside and unconditioned by ourselves. 

 Clerk-Maxwell's demon would perceive nature as some- 

 thing totally different from our nature, and to a less 

 extent this is in great probability true for the animal 

 world, and even for man in different stages of growth 

 and civilisation. The worlds of the child and of the 



1 This "being" has become known to fame as " Clerk -Maxwell's demon," 

 but it must be noted that Clerk -Maxwell supposes the being's attributes 

 "essentially finite as our own" — a peculiarity not usually associated with 

 demons. 



