Van MELSEN : "Observation and measurement thus demand qualitative differences, because they at least demand a difference in intensity of a certain quality, and also this is a qualitative difference. From this it follows that Natural Science, as empirical science, presupposes that [physical] matter is more than just quantity. Empirical science presupposes in [physical] matter a factor, which renders the quantitative relations observable, and it is this factor, that we call the qualitative."