254 THE GRAMMAR OF SCIENCE 



our knowledge it is impossible to tell whether it would or 

 would not simplify things to conceive the atom as pene- 

 trable or impenetrable. Hence, even if we go so far as 

 to give the concept atom a phenomenal existence, it will 

 not help us to understand what is meant by the assertion 

 that matter is impenetrable. 



§ 6. — Individuality does not denote Sameness in Subst7'atum 



Shall we, however, be more dogmatic still, and, denying 

 that ether is matter, assert that matter is impenetrable 

 relative to matter ? In order to give any definite answer 

 to this question we have again to pass from the perceptible 

 material group to its supposed elementary basis, the atom, 

 and to ask whether we have any reason for conceiving 

 atoms as incapable of penetrating each other. In the 

 first place, the physicist, although he has never caught an 

 atom, yet conceives it as something which is incapable of 

 disappearing — it continues to be. In the next place, if we 

 conceive it as entering into combination with a second 

 atom, although we have no reason for asserting that the 

 two atoms do not mutually penetrate, we are still com- 

 pelled, in order to describe by aid of atoms our perceptual 

 experience, to conceive that, out of the combination, two 

 separate atoms can again be obtained with the same 

 individual characteristics as the original two possessed. 

 What right have we to postulate these laws with regard to 

 atoms when atoms are, even if " real," still absolutely im- 

 perceptible to us, when we are absolutely unable to observe 

 their mutual actions ? We have exactly the same logical 

 right as we have to lay down any scientific law whatever. 

 Namely, we find that these laws as to the action of single 

 atoms, when applied to large groups of atoms, enable us 

 to describe with very great accuracy what occurs in those 

 phenomenal bodies which we scientifically symbolise by 

 groups of atoms; they enable us to construct, without 

 contradiction by perceptual experience, those routines of 

 sense-impression which we term chemical reactions. 



The hypotheses that the individual atom is both in- 



