"When in common language, therefore, we express the _value_ of any commodity, it may vary at one period from what it is at another, in consequence of eight different contingencies. 1. "From the four circumstances above stated, in relation to the commodity of which we mean to express the value. 2. "From the same four circumstances, in relation to the commodity we have adopted as a measure of value."[50] This is true of monopolized commodities, and indeed of the market price of all other commodities for a limited period. If the demand for hats should be doubled, the price would immediately rise, but that rise would be only temporary, unless the cost of production of hats, or their natural price, were raised. If the natural price of bread should fall 50 per cent. from some great discovery in the science of agriculture, the demand would not greatly increase, for no man would desire more than would satisfy his wants, and as the demand would not increase, neither would the supply; for a commodity is not supplied merely because it can be produced, but because there is a demand for it. Here then we have a case where the supply and demand have scarcely varied, or if they have increased they have increased in the same proportion; and yet the price of bread will have fallen 50 per cent. at a time too when the value of money had continued invariable. Commodities which are monopolized, either by an individual, or by a company, vary according to the law which Lord Lauderdale has laid down: they fall in proportion as the sellers augment their quantity, and rise in proportion to the eagerness of the buyers to purchase them; their price has no necessary connexion with their natural value: but the prices of commodities, which are subject to competition, and whose quantity may be increased in any moderate degree, will ultimately depend, not on the state of demand and supply, but on the increased or diminished cost of their production. CHAPTER XXIX. MR. MALTHUS'S OPINIONS ON RENT.