the minds of men, including physical inquirers, were still possessed by the old notion that reality in order to be solid and firm must con- sist of those fixed immutable things which philosophy calls substances. Changes could be known only if they could be somehow reduced to recombinations of original unchanging things. For these alone can be objects of certainty the chang- ing is as such the uncertain and only the certain and exact is knowledge. Thus apopular metaphysics, given rational for- mulation by the Greeks, and taken over into the intellectual tradition of the western world, controlled at first the inter- pretations placed upon the procedures and conclusions of ex- perimental knowing.