 In spite of these control procedures, it is clear that the derivation of a causal factor from a Millean difference test presupposes strong assumptions. In particular, it must be assumed that we are dealing with a deterministic causal structure (3) and that nothing happens uncaused (4). In our simple example, this amounts to assuming that the bacteria don’t exhibit any kind of spontaneity, in other words, their growth behavior is assumed to be determined by their genetic constitution and their environment (although most biologists are convinced that their experimental organisms have good and bad days just like themselves!).