The Wealth of Nations by Part 4 Chapter 5 Page 75

divide his capital between two different employments; to keep one part of it in his granaries and stack yard, for supplying the occasional demands of the market; and to employ the other in the cultivation of his land. But as he could not afford to employ the latter for less than the ordinary profits of farming stock, so he could as little afford to employ the former for less than the ordinary profits of mercantile stock.

Whether the stock which really carried on the business of the corn merchant belonged to the person who was called a farmer, or to the person who was called a corn merchant, an equal profit was in both cases requisite in order to indemnify its owner for employing it in this manner; in order to put his business upon a level with other trades, and in order to hinder him from having an interest to change it