Where competition prevails, price, as has just been seen, is regulated by the forces of supply and demand. Under a condition of monopoly, prices are still fixed by the law of supply and demand, but the forces of supply are controlled by the monopolist. Monopoly may, therefore, be defined as such unity of control over the supply of a commodity, whether exercised by one or more persons, as will enable the person or persons exercising such control to regulate prices. In a word, monopoly is the absence of competition. While it is usual to define monopoly as unity of control over supply, it may in some cases consist in unity of demand for a commodity. Thus where there is only one purchaser of a commodity and many producers, that purchaser may fix prices through his control of the amount demanded.