Like the rules of equality, needs, and efforts, that of productivity cannot be universally enforced in practice. It is susceptible of accurate application among producers who perform the same kind of work with the same kind of instruments and equipment; for example, between two shovellers, two machine operators, two bookkeepers, two lawyers, two physicians. As a rule, it cannot be adequately applied to a product which is brought into existence through a combination of different processes. The engine driver and the track repairer contribute to the common product, railway transportation; the bookkeeper and the machine tender co-operate in the production of hats; but we cannot tell in either case whether the first contributes more or less than the second, for the simple reason that we have no common measure of their contributions. Sometimes, however, we can compare the productivity of _individuals_ engaged in different processes; that is, when both can be removed from the industry without causing it to come to a stop. Thus, it can be shown that a single engine driver produces more railway transportation than a single track repairer, because the labour of the latter is not indispensable to the hauling of a given load of cars. But no such comparison can be made as between the whole body of engine drivers and the whole body of track repairers, since both groups are indispensable to the production of railway transportation. Again, a man can be shown to exert superior productivity because he affects the productive process at more points and in a more intimate way than another who contributes to the product in a wholly different manner. While the surgeon and the attendant nurse are both necessary to a surgical operation, the former is clearly more productive than the latter. When due allowance is made for all such cases, the fact remains that in a large part of the industrial field it is simply impossible to determine remuneration by the rule of comparative productivity. _The Canon of Scarcity_