Failure to discriminate between value and its price-expression, or symbol, has led to endless difficulty. It lies at the bottom of the naive theory that value depends upon the relation of supply and demand. Lord Lauderdale's famous theory has found much support among later economists, though it is now rather unpopular when stated in its old, simple form. Disguised in the so-called Austrian theory of final utility, it has attained considerable vogue.[172] The theory is plausible and convincing to the ordinary mind. Every day we see illustrations of its working: prices are depressed when there is an oversupply, and elevated when the demand of would-be consumers exceeds the supply of the commodities they desire to buy. It is not so easy to see that these effects are temporary, and that there is an automatic adjustment going on. Increased demand raises prices for a while, but it also calls forth an increase in supply which tends to restore the old price level, or may even force prices below it. In the latter case, the supply falls off and prices find their real level. The relation of supply to demand causes an oscillation of prices, but it is not the determinant of value. When prices rise above a certain level, demand slackens or ceases, and prices are inevitably lowered. Prices may and do fall with a decreased demand, but it is clear that unless the producers can get a price approximately equal to the value of their commodities, they will cease to produce them, and the supply will diminish or cease altogether. Ultimately, therefore, the fluctuations of price through the lack of equilibrium between supply and demand adjust themselves, and prices must tend constantly to approximate values.