The attempt to control trade and commerce was no less lofty and no less arduous.  It is perhaps still easier to stop war than to stop competition; and yet the Church made the attempt.  The Christian law of love was set against the economic law of demand and supply.  It was canonical doctrine that the buyer should take no more, and the seller offer no less, than the just price of a commodity—­a price which would in practice depend on the cost of production.  The rule for prices was also the rule for wages:  the just wage was the natural complement of the just price.  The prohibition of usury and of the taking of interest was another factor in the same circle of ideas.  If prices and wages are both to be returns for work done, and returns of an exact equivalence, then, on the assumptions which the canonists made—­that the usurer does no work, and that his loan is unproductive of any new value—­it necessarily follows that no return is due, or can be justly paid, for the use of borrowed money.  Work is the one title of all acquisition, and all acquisition should be in exact proportion to the amount of work done.  This is the basic principle, and it is the principle of the Divine Law:  In sudore frontis tuae comedes panem tuum.  Once more, therefore, and once more in an unpromising and intractable material, we find the Church seeking to enforce the unity of the Christian principle and to reduce the Many to the One.  In the same way, and from the same motive, that private war was to be banished from the feudal class in the country, competition—­the private war of commerce—­was to be eliminated from the trading classes in the towns.  Nor was the attack on competition, any more than the attack on war, so much of a forlorn hope as it may seem to a modern age.  Even to-day, custom is still a force which checks the operation of competition, and custom covered a far greater area in the Middle Ages than it does to-day.  The rent of land, whether paid in labour or in kind, was a customary rent; and in every mediaeval community the landed class was the majority.  It was an easy transition from fixed and customary rents to the fixing of just prices for commodities and services.  Lay sentiment supported clerical principle.  Guilds compelled their members to sell commodities at a level price, and in a spirit of collectivism endeavoured to prevent the making of corners and the practice of undercutting.  Governments refused to recognize the ‘laws’ of demand and supply, and sought, by Statutes of Labourers, to force masters to give, and workman to receive, no more and no less than a ‘just’ and proper wage.