The idea of a general rate of wages, as it appears in economic theory, rests upon certain broad assumptions. One of the most important of these is that there are no "differences of inborn gifts," which would lead to a limitation of the flow of labor into the upper grades, and thus lead to a separation of grades. A second important assumption is that of complete mobility of labor--no obstacles of habit, expense or ignorance to retard the flow of labor from place to place, or from industry to industry. A third assumption is the absence of combination among the workers. A fourth is that of equality of opportunity among the wage earners; and the absence of barriers of race, religion or sex. Granted these assumptions, the tendency to equality of earnings for labor demanding equal skill and effort and performed with equal efficiency is established. Competition among the workers for employment and among the employers for workmen would bring this about. Such differences of wages as would exist would arise from differences in the nature of the work performed. Thus Adam Smith wrote that "in a society where things were left to follow their natural course, where there was perfect liberty, and where every man was perfectly free both to choose what occupation he thought proper, and to change it as often as he thought proper" five circumstances would explain "a small pecuniary gain in some employments, and counter balance a great one in others." These in his words were: "First, the agreeableness or disagreeableness of the employments themselves; secondly, the easiness and cheapness, or the difficulty and expense of learning them; thirdly, the constancy or inconstancy of employment in them; fourthly, the small or great trust which must be reposed in those who exercise them; and, fifthly, the probability or improbability of success in them."[27] All such differences would be such as "equalize the attractiveness of occupations" and would be "equalizing differences."[28] If these assumptions were realized in fact, it would be correct to view the problem of wages as the study of one set of relationships that governed a basic level of wages--called the general rate of wages--with purely supplementary studies of the circumstances governing equalizing differences. The problem of wages would be a study of forces which were uniformly influential in relation to the wages of all labor. For all wages bargains would be governed by them.