Let us now sum up the theoretical differences which the alternatives of Mechanism and Emergence would make to our view of the external world and of the relations between the various sciences. The advantage of Mechanism would be that it introduces a unity and tidiness into the world which appeals very strongly to our aesthetic interests. On that view, when pushed to its extreme limits, there is one and only one kind of material. Each particle of this obeys one elementary law of behaviour, and continues to do so no matter how complex may be the collection of particles of which it is a constituent. There is one uniform law of composition, connecting the behaviour of groups of these particles as wholes with the behaviour which each would show in isolation and with the structure of the group. All the apparently different kinds of stuff are just differently arranged groups of different numbers of the one kind of elementary particle; and all the apparently peculiar laws of behaviour are simply special cases which could be deduced in theory from the structure of the whole under consideration, the one elementary law of behaviour for isolated particles, and the one universal law of composition. On such a view the external world has the greatest amount of unity which is conceivable. There is really only one science, and the various “special sciences” are just particular cases of it. This is a magnificent ideal; it is certainly much more nearly true than anyone could possibly have suspected at first sight; and investigations pursued under its guidance have certainly enabled us to discover many connexions within the external world which would otherwise have escaped our notice. But it has no trace of self-evidence; it cannot be the whole truth about the external world, since it cannot deal with the existence or the appearance of “secondary qualities” until it is supplemented by laws of the emergent type which assert that under such and such conditions such and such groups of elementary particles moving in certain ways have, or seem to human beings to have, such and such secondary qualities; and it is certain that considerable scientific progress can be made without assuming it to be true. As a practical postulate it has its good and its bad side. On the one hand, it makes us try our hardest to explain the characteristic behaviour of the more complex in terms of the laws which we have already recognised in the less complex. If our efforts succeed, this is sheer gain. And, even if they fail, we shall probably have learned a great deal about the minute details of the facts under investigation which we might not have troubled to look for otherwise. On the other hand, it tends to over-simplification. If in fact there are new types of law at certain levels, it is very desirable that we should honestly recognise the fact. And, if we take the mechanistic ideal too seriously, we shall be in danger of ignoring or perverting awkward facts of this kind. This sort of over-simplification has certainly happened in the past in biology and physiology under the guidance of the mechanistic ideal; and it of course reaches its wildest absurdities in the attempts which have been made from time to time to treat mental phenomena mechanistically.

C. D. Broad, The Mind and Its Place in Nature, London, 1937, pp. 76-77