Suppose that on another day one notices that something is wrong with the cocoa powder that is produced. It tastes as it should but it isn't powdered; instead, it is emerging from the factory in the form of solid cakes. Yet the sweet chocolate and milk chocolate are absolutely normal. this immediately suggests that there is a processor in the factory whose job is specifically to turn solid cakes into powder-called, for example, the pulverizer-and that this processor does not play any part in the production of sweet chocolate or milk chocolate, if there is such a pulverizer, and if it is not working just now, the outcome would be that sweet chocolate and milk chocolate would still be produced normally, but cocoa would still be coming out, but abnormally-as cakes rather than as powder.

This inference seems entirely reasonable, but again an alternative explanation comes fairly readily to mind. Perhaps there is no Pulverizer Module; perhaps instead there's a single machine that both presses and (when required) pulverizes, and so is needed in the making of all three products. suppose, as might seem natural, that this machine needs more electrical power to pulverize than to press, and the electricity supply to the factory has weakened, in that case, pressing will still happen but pulverizing will not-but not because the two functions are functionally distinct. Here the dissociation (cocoa powder defective yet chocolate intact) can be plausibly explained without postulating the existence of a distinct Pulverizer Module.

Finally, suppose that on yet another day a different defect emerges: Both the sweet chocolate and the milk chocolate begin to taste bitter and to be coarse in texture. The cocoa powder is just fine, however. One possible explanation for these data is that there is a processor in the factory whose job is specifically to reduce the bitterness and smooth the texture of the two types of chocolate called the concher-and that this does not play any part in the production of cocoa powder.

Data suggesting that the factory contains a Pulverizer Module and a Concher Module can not instead be explained in terms of possible effects of a weakened electricity supply because on that hypothesis one could never see bad pulverizing and good conching on one occasion, and good pulverizing but bad conching on another. A single dissociation could be explained on the electricity-supply hypothesis; but a double dissociation between the two defects cannot. thus, the hypothesis that the system in the factory contains a Pulverizer Module and a Concher Module looks strong, and until someone devises an alternative hypothesis that is also compatible with the data on the two different patterns of breakdown that have been observed, it is reasonable to conclude that the functional architecture of the factory includes a Pulverizer Module and a Concher Module.

Pulverizer Module