ABSTRACT
From the outset, the theme of this book has been that in complex systems, the behaviour of the aggregate is not the same as that of the component parts. In analysing the behaviour of a system of interacting particles in physics or a biological system the behaviour of the organism is not similar to that of either the individual particles in the one case or cells in the other. The thing to be emphasised is that there is the phenomenon of ‘self-organisation’. Somehow the components organise themselves in such a way that their collective behaviour is coherent. As I have said, there are several possibilities. One of these is that what we observe at the aggregate level corresponds to what we are led to expect by our theory. This theory is normally based on the rational behaviour of maximising individuals. The temptation is then to conclude that because the aggregate is well behaved, the individuals must, indeed, correspond to those in our theoretical models. This is, as I have already emphasised, a non sequitur. We saw this when examining the buyers on the Marseille fish market. At the aggregate level, the demand curves for fish have the nice monotone declining property that basic theory would lead us to expect, yet examination of the demand of the individual buyers reveals no such property. In this case, just because the aggregate behaves as it would if all the individuals satisfied the assumptions made about them, it is not necessarily the case that the individuals actually behave in that way. The aggregate behaviour is the result of the aggregation and not of the fact that the individuals are ‘well behaved’.
