In the previous chapters it was stated that the entropy of a system can be determined by statistical means by counting the number of micro-states in which the system may occur under the given conditions. By failing to enlarge upon this point, we have falsely given the impression that the way in which the count should be made was definitely established. Even in the simple case of the various configurations of 50 white and 50 red billiard balls (Fig. 11), counting is more ambiguous than was suggested in Chapter 2. If two white balls are interchanged, a state is reached which, to all appearances, is indistinguishable from the original state and which was thus not counted separately. However, a more thorough investigation with the help of scales and measuring instruments would certainly make it possible to distinguish two similarly coloured balls from one another. The number of visually-distinguishable configurations is thus much smaller than the number — 100! — which can be distinguished on closer investigation.