In one of the most brilliant papers in the English language Hume made it clear that what we speak of as 'causality' is nothing more than the phenomenon of repetition. When we mix sulphur with saltpeter and charcoal we always get gunpowder. This is true of every event subsumed by a causal law in other words, everything which can be called scientific knowledge. "It is custom which rules ," Hume said, and in that one sentence undermined both science and philosophy .