1) We say: the distances ⋯ by train change only in a discontinuous way. ⋯ Those of the places which can be reached by car, however, may change by steps as small as we wish, they can vary in a continuous way.

2) The output of a coal mine can change in a continuous way. ⋯ But the number of miners employed can change only discontinuously.

3) A sum of money can change only by jumps, in a discontinuous way. In America the smallest permissible change or, as we shall call it, the “elementary quantum” for American money, is one cent.

4) These indivisible steps are called the elementary quanta of the particular quantity to which they refer.

5) From this example we see how the discontinuous character of a quantity, so far regarded as continuous, can be detected by increasing the precision of our measurements.

6) If we had to characterize the principal idea of the quantum theory in one sentence, we could say: it must be assumed that some physical quantities so far regarded as continuous are composed of elementary quanta.

7) The region of facts covered by the quantum theory is tremendously great.