AdamSmithfanciedthatthereweretwocommoditiespeculiarly
fittedtoserveasameasureofvalue:corn，andlabour。Ofcorn，
hesaidthatalthoughitsvaluefluctuatesmuchfromyearto
year，itdoesnotvarygreatlyfromcenturytocentury。Thiswe
nowknowtobeanerror:corntendstoriseincostofproduction