In Chapter 3 we present the first main part of our case for Causal Mechanism, by discussing in detail apoptosis, a central biological mechanism. We examine how John Kerr and his co-workers first introduced apoptosis in 1972. We then present the most important stages in scientific research regarding apoptosis during the last decades that led to its identification as a central biological mechanism, explaining the shift from morphological descriptions to biochemical descriptions of the mechanism. We generalise the molecular definition of a pathway to arrive at a more general notion of a causal pathway. We also show that several distinctions used by biologists in order to differentiate between causal pathways and identify the genuine biological mechanisms (active vs passive, programmed vs non-programmed, physiological vs accidental) do not correspond to internal features of causal pathways, but concern an external feature, that is, the role those processes play within the organism.