The engine of growth in the models

The engine of growth in the models of sections 4 and 5 is human capital.
Within the context of these two models, human capital is simply an unobservable
magnitude or force, with certain assumed properties, that I have
postulated in order to account for some observed features of aggregative
behavior. If these features of behavior were all of the observable consequences
of the idea of human capital, then I think it would make little difference if we
simply re-named this force, say, the Protestant ethic or the Spirit of History or
just 'factor X'. After all, we can no more directly measure the amount of
human capital a society has, or the rate at which it is growing, than we can
measure the degree to which a society is imbued with the Protestant ethic.