 The law of classification of the sciences also has a historical aspect: it gives us the order in which the sciences develop. For example, astronomy requires mathematics, and chemistry requires physics. Each science thus rests upon the one that precedes it. As Comte puts it, the higher depends on the lower, but is not its result. The recognition of an irreducible diversity already contains a disavowal of reductionism (in Comte’s wording: ‘materialism’), which the classification allows one to make explicit. The positivist clearly sees that the tendency towards reductionism is fed by the development of scientific knowledge itself, where each science participates in the evolution of the next; but history also teaches us that each science, in order to secure its own subject matter, has to fight invasions by the preceding one. ‘Thus it appears that Materialism is a danger inherent in the mode in which the scientific studies necessary as a preparation for Positivism were pursued. Each science tended to absorb the one next to it, on the grounds of having reached the positive stage earlier and more thoroughly.’ (1851, v. 1, 50; E., v. 1, 39)