Capitalist firms produce commodities, that is a good or service
produced for sale only. Every commodity has a use-value for
people. That means they are useful to someone otherwise they
could not be sold. They also contain a value. What is it and how
can it be shown? If we leave the use of money out for the time
being, commodities, when they are exchanged, fall into certain
proportions. For example: A pair of shoes = 10 meters of cloth =
1 watch = 3 bottles of whisky. This simple example shows that
the exchange value of different commodities expresses something
contained in them. But what makes a pair of shoes = 10 yards of
cloth = 3 bottles of whisky? The only thing in common is they
are all products of human labour. The amount of human labour
contained in a commodity is expressed in time: weeks, days,
hours, minutes. To go back to the example: all these commodities
can be expressed in terms of their common factor, labour-time. 5
hours (labour) worth of shoes 5 hours (labour) worth of watches
5 hours (labour)