Adam Smith, Wealth of Nations,   CHAPTER X,

   Of Wages and Profit in the different Employments of Labour and Stock

    Fourthly, the wages of labour vary accordingly to the small or great trust which  must be reposed in the workmen.  The wages of goldsmiths and jewellers are everywhere superior to those of many  other workmen, not only of equal, but of much superior ingenuity, on account of  the precious materials with which they are intrusted.   We trust our health to the physician: our fortune and sometimes our life and reputation to the lawyer and attorney. Such confidence could not safely be  reposed in people of a very mean or low condition. Their reward must be such,  therefore, as may give them that rank in the society which so important a trust  requires. The long time and the great expense which must be laid out in their education, when combined with this circumstance, necessarily enhance still  further the price of their labour.   When a person employs only his own stock in trade, there is no trust; and the  credit which he may get from other people depends, not upon the nature of his  trade, but upon their opinion of his fortune, probity, and prudence. The  different rates of profit, therefore, in the different branches of trade, cannot arise from the different degrees of trust reposed in the traders.