THE REAL WORLD 377 



tain that we are justified in regarding these particles as real in a sense 

 not essentially different from the usual meaning of the word. 



I do not know what Born takes to be "the usual meaning" of real. 

 But the human search for invariants, conducted within the boundary 

 condition imposed by the demand for an over-all system of high 

 correlative index, cannot ensure its own success. The materialization 

 of invariants in a great variety of "projections" of subatomic particles 

 is then a genuine discovery. But now, if ordinary physical objects are 

 mythical, how much less even than legendary will be those sub- 

 atomic particles. Yet there is nothing mythical about the invariance 

 they display. That is a fact demanding explanation, a fact inexpli- 

 cable in terms of myths, a fact explained if (with Born) we suppose 

 that the conceptual entities do have some kind of real cognates. 



Born's "simultaneous observation of the shadows on several dif- 

 ferent planes" yields us a cross-fix that assures us that we have indeed 

 found an invariant. Just so, when concordant values of atomic con- 

 stants emerge from wholly distinct bodies of data, the cross-fix so 

 established is, as Born notes, of great significance. 



The kinetic interpretation of the deviations from Boyle's law leads to 

 an estimate of the size of the molecules, which is confirmed by a quite 

 different set of phenomena, the irreversible processes of heat conduc- 

 tion, viscosity, diffusion. Many concepts first introduced in a theoreti- 

 cal way, like velocity distribution, free path, etc., have been con- 

 firmed and determined by direct measurements. The fluctuations pre- 

 dicted by the kinetic theory are observable in many ways, through the 

 Brownian motion, the blue color of the sky, etc. . . . the kinetic 

 theory leads to definite properties of the molecules, weight, size, 

 shape (degrees of freedom), mutual interaction. A small number of 

 molecular constants determines an unlimited number of phenomeno- 

 logical properties, in virtue of the molecular hypothesis. Therefore 

 each new property predicted is a confirmation of the molecular 

 hypothesis. 



A confirmation perhaps particularly impressive is Einstein's success 

 in predicting on theoretical grounds the yet-to-be-discovered law of 

 a Brownian motion which (though discovered some 75 years earlier 

 by the botanist B. Brown) Einstein did not even know to exist as a 

 phenomenon. To be sure, no aggregate of such confirmations can 

 demonstrate the real existence of particles objectively like those we 



